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