In Phabricator, drop PhutilClassMap caches after loading additional libraries

Summary:
Depends on D16755. Right now, we build a setup check map (to run preflight checks), then later load libraries.

This means any checks included in third-party libraries don't get added to the map, and no longer run.

(These are rare, but Phacility has a couple).

Instead, delete the caches after loading extra libraries.

Test Plan: With this and D16755, re-ran setup checks and saw Phacility setup checks run.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D16756