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