Disable use of pip install --python on Gentoo debundled pip

[mgorny]

In the debundled pip version distributed on Gentoo systems, pip install --python is nonfunctional since it expects all of pip's dependencies to be present in the (empty) virtual environment. Detect this case by the removal of pip._vendor package, and use the "no valid outer pip" codepath.

Bug: https://bugs.gentoo.org/934922

[mgorny]

Looks like distutils import hack from setuptools is causing trouble here. Any suggestion how to deal with that?

[henryiii]

I don't think this will fix it, but I'd not do import if you aren't going to use it later; use importlib.util.find_spec("pip._vendor") instead, it's much faster. To me, isn't it a bit broken if pip install --python can't be used? You can target a completely empty environment with pip install --python normally.

[mgorny]

Yes, it is. Unfortunately, I don't see how to fix it — in the end, the --python is a huge hack, and we find unbundling dependencies more important than, well, an option that we didn't even knew existed before build started using it.

[mgorny]

I think the remaining failures are flaky tests.

[henryiii]

What's the warning, out of curiosity?

[mgorny]

What's the warning, out of curiosity?

Basically this:

>>> import distutils
>>> import pip
/usr/lib/python3.10/site-packages/_distutils_hack/__init__.py:30: UserWarning: Setuptools is replacing distutils. Support for replacing an already imported distutils is deprecated. In the future, this condition will fail. Register concerns at https://github.com/pypa/setuptools/issues/new?template=distutils-deprecation.yml
  warnings.warn(

[henryiii]

the --python is a huge hack

Pip is full of hacks - the build-isolation support is a mess, for example. IMO using --python instead of installing pip into every single environment is much more reasonable, and the main reason people don't use it is because it's a newer feature and it's not the way we used to work. uv always works this way, and it makes more sense.

Regardless, though, this seems fine.

I'll try to fix the univeral= warning soon.