7417: Fix bundler/inline warning: method redefined; discarding old root r=deivid-rodriguez a=fatkodima

Closes #6167

Co-authored-by: fatkodima <fatkodima123@gmail.com>

7416: Don't use insecure temporary directory as home directory r=deivid-rodriguez a=fatkodima

Closes #6501 

Co-authored-by: fatkodima <fatkodima123@gmail.com>

7424: Drop unnecessary `tempfile` require r=deivid-rodriguez a=deivid-rodriguez

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

The `tempfile` dependency was dropped in [1], but the require was left in there. This could cause gem conflicts because `tempfile` requires `tmpdir` which requires `fileutils`, which loads the default gem instead of our namespaced version, and that could cause fileutils version mismatches and code overriding warnings.

[1]: https://github.com/bundler/bundler/commit/4a37d66f3f222739178d798b30fb135f2429fe45

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

My fix is to drop the unnecessary require.


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

The `tempfile` dependency was dropped in [1], but the require was left
in there. This could cause gem conflicts because `tempfile` requires
`tmpdir` which requires `fileutils`, which loads the default gem instead
of our namespaced version, and that could cause fileutils version
mismatches and code overriding warnings.

[1]: https://github.com/bundler/bundler/commit/4a37d66f3f222739178d798b30fb135f2429fe45

7393: Make 'bundle add' cache newly added gems when needed r=deivid-rodriguez a=andrehjr

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

The problem was that when calling `bundle add` with cache_all as true, I have to call an additional `bundle install` to vendor gems.

### What was your diagnosis of the problem?

My diagnosis was that a call to Bundler.load.cache was missing. For example Bundler::CLI::Update does the same after installing gems.

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

My fix was to call Bundler.load.cache when `Bundler.app_cache.exist? ` 

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

I chose this fix because it looks like this makes the behavior consistent with other commands.

I should also say that, as this is my first PR here, I'm not sure that this is the best solution, and it seems an easy fix for #7384. 

Co-authored-by: André Luis Leal Cardoso Junior <andrehjr@gmail.com>

7394: Add inline RDoc documentation r=hsbt a=zverok

Since Bundler became part of the standard library, it renders in Ruby's docs. Unfortunately, what renders there...  is not really helpful: https://docs.ruby-lang.org/en/master/Bundler.html

I've added rudimentary documentation for `Bundler` module and two of its most user-facing methods to solve this problem at least partially.

Co-authored-by: zverok <zverok.offline@gmail.com>

Bundler no longer needs to be in the `$LOAD_PATH` once a version of
itself has been loaded.

In any case, the `Metadata` source takes care of adding `bundler` to the
index:

https://github.com/bundler/bundler/blob/e70643c1be3a4417bd537d7e63470265465e693e/lib/bundler/source/metadata.rb#L13-L30

and then that spec is added to the resolution here:

https://github.com/bundler/bundler/blob/e70643c1be3a4417bd537d7e63470265465e693e/lib/bundler/definition.rb#L180-L184

And from the resulting set of specs, the `$LOAD_PATH` is setup:

https://github.com/bundler/bundler/blob/e70643c1be3a4417bd537d7e63470265465e693e/lib/bundler/runtime.rb#L25-L38

So `bundler` will be present in the `$LOAD_PATH` anyways, and the lines
being removed here will never be useful.