With this PR I would like to facilitate the implementation of the venv
layouts because we can in theory take the `srcs` and the `data` within
the `py_library` and then use the `expand_template` to write the extra
Python files if the namespace_pkgs flag is enabled.

The old Python code has been removed and the extra generated files are
written out with `bazel_skylib` `copy_file`.

The implicit `namespace_pkg` init files are included to `py_library`
if the `site-packages` config flag is set to false and I think this
may help with continuing the implementation, but it currently is still
not working as expected (see comment).

Work towards #2156

The migration effort has stalled and we closed the initiative.

#1360