Add a method to drop all PhutilClassMap caches

Summary:
The addition of "preflight" setup checks caused us to generate a setup check cache early, and stop running third-party setup checks (these are rare, but we have a couple in Phacility stuff).

Add a method so we can drop caches explicitly.

The next change drops caches in Phabricator after loading libraries.

Test Plan: See next diff.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D16755