go_path rule does not support go_library using embedsrcs containing a directory

[cmfcmf]

If you define a go_library that uses embedsrcs referring to a directory (i.e., by either containing the name of a local directory, or by referring to a label of a rule that creates a directory, like copy_to_directory), then a go_path rule (transitively) referring to that library fails to build with an error from the go_path.go binary complaining about not being able to copy a directory.
This is because the go_path.go binary wrongly assumes that all paths in the manifest are files, which is not the case when specifying a directory like described above.

I have a PR ready with a proposed fix, but I am not a Bazel expert, so I am unsure whether my fix is correct: #4401