7313: Fix a couple of typos r=deivid-rodriguez a=deivid-rodriguez

### What was the end-user problem that led to this PR?

The problem was typos.

### What is your fix for the problem, implemented in this PR?

My fix is to correct typos.

Co-authored-by: David Rodríguez <deivid.rodriguez@riseup.net>

7312: Remove old rubies stuff no longer needed r=hsbt a=deivid-rodriguez

### What was the end-user problem that led to this PR?

The problem was that was still some code specific to old rubies we no longer support.

### What is your fix for the problem, implemented in this PR?

My fix is to remove that code.

Co-authored-by: David Rodríguez <deivid.rodriguez@riseup.net>

7274: Fix more leaks to default copy of bundler r=hsbt a=deivid-rodriguez

### What was the end-user problem that led to this PR?

The problem was that in some places, it was still possible to end up requiring files in a different copy of bundler (the default copy). I noticed this when I removed a rubygems monkeypatch from the test suite that was preventing the default copy of bundler from being activated when requiring files.

This thing:

https://github.com/bundler/bundler/blob/e1c518363641208429f397170354054b3d28effd/spec/support/hax.rb#L15-L20

### What was your diagnosis of the problem?

My diagnosis was that I should use relative requires wherever they were missing.

### What is your fix for the problem, implemented in this PR?

My fix is to remove the rubygems hack, migrate the rest of the internal requires to be relative, and also introduce some hacks on our specs to make sure we never load the incorrect copy of bundler.

I think this PR should fix the issues in https://github.com/rubygems/rubygems/pull/2863.


Co-authored-by: David Rodríguez <deivid.rodriguez@riseup.net>

7302: Bundler displays a duplicate gem entries warning even if gems only appear once per group r=hsbt a=davidstosik

### What was the end-user problem that led to this PR?

I have a Gemfile where some gems appear in two distinct groups. When I run Bundle, the following warning is displayed:

> Your Gemfile lists the gem #{gem} more than once.
> You should probably keep only one of them.
> Remove any duplicate entries and specify the gem **only once (per group)**.
> While it's not a problem now, it could cause errors if you change the version of one of them later.                                                           

Example Gemfile:
```rb
source "https://rubygems.org"

group :production do
end

group :development do
  gem "rake"
end

group :ci, optional: true do
  gem "rake"
end
```

### What was your diagnosis of the problem?

I think the message is misleading, because it shows even if all gems are specified **only once per group**.

### What is your fix for the problem, implemented in this PR?

I removed the _(per group)_ mention from the warning message in order to prevent confusion.

### Why did you choose this fix out of the possible options?

I chose this fix because it was the simplest way to bring your attention to the problem.

If Bundler's real intent is to allow the same gem to appear in multiple groups, as long as it does not appear more than once per group, then my suggestion is wrong, and instead, the logic around the warning message needs to be fixed.

Co-authored-by: David Stosik <david.stosik+git-noreply@gmail.com>

7309: Fix `bundle doctor` command r=hsbt a=deivid-rodriguez

### What was the end-user problem that led to this PR?

The problem was that `bundle doctor` crashes on very simple usages, like this:

````
$ bundle doctor
The Gemfile's dependencies are satisfied
--- ERROR REPORT TEMPLATE -------------------------------------------------------
# Error Report

## Questions

Please fill out answers to these questions, it'll help us figure out
why things are going wrong.

- **What did you do?**

  I ran the command `/home/deivid/.rbenv/versions/2.6.3/bin/bundle doctor`

- **What did you expect to happen?**

  I expected Bundler to...

- **What happened instead?**

  Instead, what happened was...

- **Have you tried any solutions posted on similar issues in our issue tracker, stack overflow, or google?**

  I tried...

- **Have you read our issues document, https://github.com/bundler/bundler/blob/master/doc/contributing/ISSUES.md?**

  ...

## Backtrace

```
Errno::ENOENT: No such file or directory - /home/deivid/Code/playground/bundler/chcek/.bundle/ruby/2.6.0/bundler
  /home/deivid/.rbenv/versions/2.6.3/lib/ruby/2.6.0/find.rb:43:in `block in find'
  /home/deivid/.rbenv/versions/2.6.3/lib/ruby/2.6.0/find.rb:43:in `collect!'
  /home/deivid/.rbenv/versions/2.6.3/lib/ruby/2.6.0/find.rb:43:in `find'
  /home/deivid/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/bundler-2.1.0.pre.1/lib/bundler/cli/doctor.rb:105:in `each'
  /home/deivid/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/bundler-2.1.0.pre.1/lib/bundler/cli/doctor.rb:105:in `check_home_permissions'
  /home/deivid/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/bundler-2.1.0.pre.1/lib/bundler/cli/doctor.rb:83:in `run'
  /home/deivid/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/bundler-2.1.0.pre.1/lib/bundler/cli.rb:652:in `doctor'
  /home/deivid/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/bundler-2.1.0.pre.1/lib/bundler/vendor/thor/lib/thor/command.rb:27:in `run'
  /home/deivid/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/bundler-2.1.0.pre.1/lib/bundler/vendor/thor/lib/thor/invocation.rb:126:in `invoke_command'
  /home/deivid/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/bundler-2.1.0.pre.1/lib/bundler/vendor/thor/lib/thor.rb:387:in `dispatch'
  /home/deivid/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/bundler-2.1.0.pre.1/lib/bundler/cli.rb:26:in `dispatch'
  /home/deivid/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/bundler-2.1.0.pre.1/lib/bundler/vendor/thor/lib/thor/base.rb:466:in `start'
  /home/deivid/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/bundler-2.1.0.pre.1/lib/bundler/cli.rb:17:in `start'
  /home/deivid/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/bundler-2.1.0.pre.1/exe/bundle:30:in `block in <top (required)>'
  /home/deivid/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/bundler-2.1.0.pre.1/lib/bundler/friendly_errors.rb:123:in `with_friendly_errors'
  /home/deivid/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/bundler-2.1.0.pre.1/exe/bundle:22:in `<top (required)>'
  /home/deivid/.rbenv/versions/2.6.3/bin/bundle:23:in `load'
  /home/deivid/.rbenv/versions/2.6.3/bin/bundle:23:in `<main>'
```

## Environment

```
Bundler       2.1.0.pre.1
  Platforms   ruby, x86_64-linux
Ruby          2.6.3p62 (2019-04-16 revision 67580) [x86_64-linux]
  Full Path   /home/deivid/.rbenv/versions/2.6.3/bin/ruby
  Config Dir  /home/deivid/.rbenv/versions/2.6.3/etc
RubyGems      3.1.0.pre1
  Gem Home    /home/deivid/Code/playground/bundler/chcek/.bundle/ruby/2.6.0
  Gem Path    /home/deivid/Code/playground/bundler/chcek/.bundle/ruby/2.6.0
  User Home   /home/deivid
  User Path   /home/deivid/.gem/ruby/2.6.0
  Bin Dir     /home/deivid/Code/playground/bundler/chcek/.bundle/ruby/2.6.0/bin
Tools         
  Git         2.22.1
  RVM         not installed
  rbenv       rbenv 1.1.2
  chruby      not installed
```

## Bundler Build Metadata

```
Built At          2019-08-15
Git SHA           91f91a1ad
Released Version  false
```

## Bundler settings

```
gem.test
  Set for the current user (/home/deivid/.bundle/config): "rspec"
gem.mit
  Set for the current user (/home/deivid/.bundle/config): true
gem.coc
  Set for the current user (/home/deivid/.bundle/config): true
default_cli_command
  Set for the current user (/home/deivid/.bundle/config): "install"
path_relative_to_cwd
  Set for the current user (/home/deivid/.bundle/config): true
path
  Set for your local app (/home/deivid/Code/playground/bundler/chcek/.bundle/config): ".bundle"
```

## Gemfile

### Gemfile

```ruby
source "https://rubygems.org"

gem "rake", "12.3.2"

gem "byebug", "~> 11.0"
```

### Gemfile.lock

```
GEM
  remote: https://rubygems.org/
  specs:
    byebug (11.0.1)
    rake (12.3.2)

PLATFORMS
  ruby

DEPENDENCIES
  byebug (~> 11.0)
  rake (= 12.3.2)

BUNDLED WITH
   2.1.0.pre.1
```

--- TEMPLATE END ----------------------------------------------------------------

Unfortunately, an unexpected error occurred, and Bundler cannot continue.

First, try this link to see if there are any existing issue reports for this error:
https://github.com/bundler/bundler/search?q=No+such+file+or+directory+-+%2Fhome%2Fdeivid%2FCode%2Fplayground%2Fbundler%2Fchcek%2F.bundle%2Fruby%2F2.6.0%2Fbundler&type=Issues

If there aren't any reports for this error yet, please create copy and paste the report template above into a new issue. Don't forget to anonymize any private data! The new issue form is located at:
https://github.com/bundler/bundler/issues/new
````

### What was your diagnosis of the problem?

My diagnosis was that `Bundler.home` is not the folder this command should use, because that folder is in reality the "home" for git gems and bundler plugins, not the home for the whole bundle. So it sometimes doesn't exist and the command crashes.

### What is your fix for the problem, implemented in this PR?

My fix is to use the proper "home".

### Why did you choose this fix out of the possible options?

I chose this fix because it's better than rescuing the error. It was an unexpected error, so we should fix it.

Fixes #6820 
Closes #6824.


Co-authored-by: David Rodríguez <deivid.rodriguez@riseup.net>