Declare 'google' namespace when importing Python protobuf via Bazel.

`python/google/__init__.py` declares the top-level 'google' namespace so that
`google.protobuf` can be imported alongside other Google Python modules like
`google.auth`.

This works well when installing protobuf via Pip, but the Bazel `//:python_srcs`
rule doesn't include this file in its `srcs`. Bazel implicitly creates an empty
`google/__init__.py`, which does *not* set up a namespace. The result is that
consuming Python protobuf via Bazel breaks all other Google Python libraries.

This fixes #4658.

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