runfiles: add pathlib.Path or importlib.Traversable API

[rickeylev]

The current runfiles library accepts runfile strings and returns file system strings.

This works fine for looking up specific resources, but can break when trying to list resources in a directory. The particular case is when a runfiles manifest is used without runfiles being materialized. In such a case, two problems occur: (1) there is no actual directory, and (2) files logically part of a runfiles directory may be in different underlying file system directories.

To fix this, we should have an API that returns an object that represents a path instead of the underlying filesystem path. The two candidates in the stdlib are pathlib.Path and importlib.Traversable. Traversable has a subset of Path methods.

IMHO, I think Path is slightly better -- Traversable is specific to the import system, while runfiles aren't.

Related slack discussion: https://bazelbuild.slack.com/archives/CDCMRLS23/p1758750938143029