poc: foreign pip

[neersighted]

This is a take on an alternate solution to our current embedded/in-venv pip woes: use a 'foreign' pip installed in Poetry's env to manage files in the project venv.

This works, except for scripts as --target does not install scripts properly, and --prefix cannot be used with the wheel platform overrides. However, if pip grows support for --prefix instead of --target in our use case, this could be a viable transitional approach for 1.2.

Tests are going to be totally busted as I have not upgraded any of them for the new code yet.

[neersighted]

After discussion with pip devs, https://github.com/pypa/pip/pull/11320 is merged and will be released soon, and will allow this to go forward without any of the drawbacks (having to gather wheel tags/not getting scripts) as we can just pass the path to the target interpreter to pip.

[Secrus]

Since we now have installer, I'd rather go towards using installer rather than subprocessing to pip. The ultimate goal should be to remove all calls to pip.

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