Starlarkify Python's build flags

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](https://docs.google.com/document/d/1h3rGVM8SpPOtxzaq0fZwcUsfRhNlNF5PNsmRe3X68gA/)
- [Starlarkify native Bazel flags](https://docs.google.com/document/d/1yOvi4hVV7Ja32ocwVb4lsEUnijftk8nilXPncYm-BH8/edit?tab=t.0#heading=h.5mcn15i0e1ch) (proposal)
- [Starlarkify native Bazel flags](https://github.com/bazelbuild/bazel/discussions/26521) (GitHub discussion)