add pathlib to third-party

Summary:
We need a good `glob()` function for the upcoming `files:` field. It turns out
that git's native globbing doesn't work very well; ** can't match the root
directory for example. Pathlib has an excellent glob, but it's only
standardized in 3.4. Add pathlib to third-party, for compatibility with 3.2 and
3.3. Now we can start using it instead of `os.path` throughout the codebase.

Test Plan: `scripts/validate_third_party.sh`

Reviewers: sean

Differential Revision: https://phabricator.buildinspace.com/D29