Use rules_python in Bazel builds

[mhucka]

The consensus is that the right way to build Python software with Bazel is to use bazelbuild/rules_python. Bazel doesn't see changes in the Python environment installed by pip or elsewhere; it's only aware of things that are "imported" into it.

Doing this will take some work in figuring out how to avoid replicating the contents of requirements.txt in the Bazel files, so that we maintain one source of truth for the Python dependencies.