Update the strip prefix logic for python libraries

[vihangm]

The strip prefix logic to generate the imports for py_library rules was inaccurate.

Lets say you have a proto file at a/b/c/d/e/f.proto.
If stripPrefix is a/b/c then imports should be ../.. since that will
ascend the import dir from the current dir e two levels to c which will then
make from d.e import f_py_pb2 work.

This updates the broken logic to implement this correct logic and adds a test to assert the same.

Fixes #362

[vihangm]

@pcj Thanks for adding support for strip_import_prefix and python libraries in the latest release. I upgraded and tried to use the feature but noticed a bug with how the ImportAttr was being calculated. This PR fixes the same.

[pcj]

@vihangm Looks good. I don't personally use strip_import_prefix so the support has always been long delayed or incomplete. Appreciate your contribution.