New PEP: Wheel Variants: Package Format

Following the suggestions given in the previous PEP 817 thread [1], we
have decided to split PEP 817 into a series of smaller PEPs, with the
hope that this will make it easier to comprehend the concept and discuss
it.

This is the first split PEP, that specifically focuses on the low-level
details necessary for variants wheels to work, that is:

- adding variant label to the filename
- storing variant properties in the file
- exposing variants on the index
- ordering/selecting variants
- introducing variant-conditional dependencies via environment markers
- exposing variant wheels in `pylock.toml`

The PEP keeps variant properties abstract, deferring their governance
and determining their compatibility to a subsequent PEP, along with
building wheels. We've also significantly cut motivation down (the
original is kept in PEP 817 for reference). We've tried to make the
"specification" part easier to comprehend, and removed the duplicate
"rationale-overview", in favor of a more focused "rationale" section.

Compared to the previous iteration of PEP 817, we've also corrected the
variant ordering algorithm to handle corner cases better.

[1] https://discuss.python.org/t/pep-817-wheel-variants-beyond-platform-tags/105860

Signed-off-by: Michał Górny <mgorny@quansight.com>
Co-authored-by: Jonathan Dekhtiar <jonathan@dekhtiar.com>
Co-authored-by: Konstantin Schütze <konstin@mailbox.org>
Co-authored-by: Ralf Gommers <ralf.gommers@gmail.com>

Author: 4 people