[bazel/py] Omit google/__init__.py from the Protobuf runtime.

[dlj-NaN]

Since google is a Python namespace package, the google/__init__.py
file should be omitted to avoid collisions. For example, its presence
may cause other Pip packages in the google namespace not to be found.

This change narrows which __init__.py files are included in the
//:protobuf_runtime target to include only those under
google/protobuf. It also moves their canonical inclusion to the
internal //:python_srcs rule, eliminating the dual specification in
python_protobuf.extra_srcs.

This is the difference in files, lexicographically ordered:

$ bazel cquery 'labels(srcs, kind(py_library, deps(:protobuf_python)))' | sort > /tmp/deps-{before,after}.txt
$ diff /tmp/deps-{before,after}.txt
1,5d0
< //:python/compatibility_tests/v2.5.0/tests/__init__.py (null)
< //:python/compatibility_tests/v2.5.0/tests/google/__init__.py (null)
< //:python/compatibility_tests/v2.5.0/tests/google/protobuf/__init__.py (null)
< //:python/compatibility_tests/v2.5.0/tests/google/protobuf/internal/__init__.py (null)
< //:python/google/__init__.py (null)
51d45
< //:python/protobuf_distutils/protobuf_distutils/__init__.py (null)

This seems like a desirable change, since it avoids polluting other
packages, too.

This is conceptually similar to #7877, but for Bazel.

[dlj-NaN]

I'm not sure if this actually warrants release notes, but we could say something like "we don't break the google package namespace anymore in Python builds under Bazel."