Better handling of generated python files

Previously this integration would fail with an error message including something like "darwin-fastbuild is not a valid Python package name" if generated files were included in a py_* target.

This was most prominently a blocker on using this integration with python_proto_library, because the Protobuf-generated Python modules have a Bazel File.root of bazel-out/<platform>/bin, where <platform> is a hyphenated string like "darwin-fastbuild".