Python: grpc's setup.py may contain unrelated files

[jmuk]

@mvashishtha noticed a bug on googleapis/artman, running PythonGrpcClientPipeline twice will cause an error. Without cleanup, the generated directory may contain several unrelated files, and therefore packman lists unrelated files in setup.py and then causes error.

setup.py shouldn't catch those unrelated files, we will need some heuristics of doing so.

[jmuk]

cc: @geigerj

[geigerj]

This comes specifically from packman's declaration of namespace packages in setup.py.

It seems somewhat odd that packman declares all possible packages as namespace packages. One non-heuristic based option might be to restrict the possible declared namespace packages to a whitelist passed as a command-line argument.

If we actually want the behavior of declaring all possible packages as namespace packages, then perhaps packman should inspect the directories to ensure they contain an init.py before marking them as namespace packages.

[jmuk]

setup.py.mustache simply lists nsPackages (https://github.com/googleapis/packman/blob/master/templates/python/setup.py.mustache) and nsPackages are generated at https://github.com/googleapis/packman/blob/master/lib/packager.js#L173

We should fix the latter.