Fixes #2404

Fixes #2404

Fixes bazel-contrib#2404

…azel-contrib#2406)

Before this PR we would shell out to `uname` on UNIX systems to get the
`arch`
of the toolchain - on Windows we would not need to do it because there
used to
be only a single Windows platform. With this change we can correctly
support
the resolution of the python interpreter on various platforms and I have
also
added an env variable to customize the selection, so that users can use
`musl`
or a `freethreaded` interpreter if they wish.

As part of this change, I have restricted visibility of the config
settings
used in the toolchain alias repo so that we are creating fewer targets.
This is
a very good time to do this before `1.0.0`.

Fixes bazel-contrib#2145
Work towards bazel-contrib#2276
Work towards bazel-contrib#2386
Work towards bazel-contrib#1211 to unblock bazel-contrib#2402
Work towards bazel-contrib#1361

---------

Co-authored-by: Richard Levasseur <richardlev@gmail.com>

With this and the previously landed bazel-contrib#2406 users can use this in `sdist`
or `whl`-only setups alike. The PR itself is very simple and just adds
`musl` toolchains.

Whilst at it also move the `WhlLibc` flag out of the `pypi` namespace
since it is also used for `toolchain` matching now.

Fixes bazel-contrib#1211