Add a function to find src labels in a specific directory

The existing api restricts search to the directory of the currently
visited soong module. Create a new function that allows search in any
directory. Internally it calls TransformSubpackagePath so the returned
labels will be resolved to respect package boundaries.

The use case for this is to support proto.include_dirs of cc_*, java_*,
python_*

Test: Added a unit test that uses a mock for GlobWihDeps

Change-Id: I75fe3ca7de88b163f456190049a22c62f91c9a2e