Use rules_python to fetch the bazel-runfiles pip package, we can later migrate other checked-in Python deps to rules_python as well.

On linux and macOS, we are using a hermetic Python toolchain fetched by rules_python, which helped avoid issues with older system installed Python version (<=3.6) on centos7 and ubuntu1804.

On windows, the hermetic Python toolchain doesn't work with `pywin32` pip package, therefore we still use the auto-detected python toolchain.

For bootstrapping test, we need to use the auto-detected python toolchain to avoid downloading Python.

Working towards: https://github.com/bazelbuild/bazel/issues/18957

RELNOTES:
PiperOrigin-RevId: 553731631
Change-Id: I605399067e21653cab707694810de24fd2109c41