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)
<...