Implement PHP-mode find.
Summary: Implement finding files via PHP functions so that find() works transparently in environments without the command find.
Test Plan: Ensure unit tests pass. Intentionally break the condition that checks if the OS is Windows to test the PHP-based find.
Reviewers: epriestley
Reviewed By: epriestley
CC: aran, Korvin
Differential Revision: https://secure.phabricator.com/D5132