Starlarkify Python's build flags

[gregestren]

As part of Starlarkification, we plan to move all Python-related build flags defined in Bazel core code into rules_python.

This:

• gives rules_python owners more complete control over Bazel's Python support
• reduces the need for bazel / rules_python release synchronization
• saves rules_python contributors from having to know how to patch Bazel.
• saves Bazel devs from having to know the details of Python

This issue tracks the effort.

Also see:

• Starlarkify Python flags
• Starlarkify native Bazel flags (proposal)
• Starlarkify native Bazel flags (GitHub discussion)

[gregestren]

I'm happy to have this assigned to me!

[rickeylev]

FYI: I've added a CI job that uses bazel rolling. It won't shorten the iteration cycle, but you'll be able to more commit changes here with more confidence they work and won't regress. If they do, we'll see it in typical PRs. The CI job is non-blocking, so we can still merge, but it'll be much more visible.