R1:224c4692ee0e

Add a cache purger for builtin files

Summary: Fixes T12859.

Test Plan:
- Loaded Diffusion builtin icons before recent updates, saw cached builtins.
- Ran `bin/cache purge --caches builtin-file`.
- Reloaded, saw new files.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12859

Differential Revision: https://secure.phabricator.com/D18147