Add an externals/includes directory

Summary: Add an externals/includes directory that will be included as a part of PHP's include_path. Arcanist already has similar functionality. Without this, it is hard to invoke the scripts/update_compat_info.php script, which was moved from rARC to rPHU in D9813.

Test Plan: N/A

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9821