* Drop bundler 1 stuff from tests.
* Move all feature flags to bundler 3 (like they are in 2-0-stable) and
get them tested.

7007: Remove lockfile incompatibility created by the `lockfile_uses_separate_rubygems_sources` setting r=deivid-rodriguez a=deivid-rodriguez

This is more of a question PR, I created this patch to try it out and try to understand, not necessarily get it merged.

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

The problem was that once we enable the `lockfile_uses_separate_rubygems_sources` setting, all lockfiles in the world will become incompatible with the previous version. Actually, not necessarily incompatible, but bundler will reorder the sections when the setting is enabled, that will generate churn lock file diffs, and _maybe_ some confusion / merge conflicts, and so on.

### What was your diagnosis of the problem?

My diagnosis was that maybe this is not necessary. I read over the issues where this setting was added and what I understood is that previously if a Gemfile specified multiple rubygems sources, they would all get merged together and that's dangerous because it's not deterministic from which source each gem will be picked up, and that could be maliciously exploited. So now each source gets its own separate section. However, how does that affect the ordering of the sections? I don't think it should affect it?

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

My fix is to change the `lock_sources` method so that both code branches (`lockfile_uses_separate_rubygems_sources == true`, and `lockfile_uses_separate_rubygems_sources == false`) result in the same ordering of the source sections.

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

I chose this fix because I _think_ it keeps the setting doing the same thing, but also keeps lock file compatibility.


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

The `lockfile_uses_separate_rubygems_sources` was causing a lockfile
incompatibility but in my opinion, this incompatibility is not necessary
in the general case.

This allows installing plugins when the bundler is frozen, since plugins are not locked anyways

6749: Add local git repository source option (`--local_git`) to plugin installation r=indirect a=indirect

Reopening #6338 to close #5446.

Co-authored-by: Saverio Miroddi <saverio.pub2@gmail.com>

I don't know why it was there, and it makes things more complicated with
dealing and running assertions on ARGV.