`pip install` completely ignores existing source builds and installed dependencies

I suspect this has to do with #4619.

I am on the latest master, and now every time I call pip install -v -e . two things happens:

1. These packages keeps being reinstalled despite I already have them in my env: setuptools, wheel, Cython, fastrlock
2. All modules are re-cythonized and recompiled from scratch, despite they’ve been built and nothing has changed

I will try to build in a fresh env to see if something is wrong with my current env. But it’s better to be confirmed independently.

cc: @kmaehashi @emcastillo

[kmaehashi]

I skimmed the pip code, seems pip always install requirements from pyproject.toml https://github.com/pypa/pip/blob/baaf66f70d84cf3367d5b43fec16762ca9703810/src/pip/_internal/distributions/sdist.py#L61-L64 although requirements from setup_requires are installed only when it is missing in the current environment: https://github.com/pypa/pip/blob/baaf66f70d84cf3367d5b43fec16762ca9703810/src/pip/_internal/distributions/sdist.py#L90-L95

[jakirkham]

There’s a bunch of things conda-build does to configure pip. May be worth trying these. In particular I think PIP_NO_BUILD_ISOLATION may be relevant, but the others may be relevant as well