internal: Fix caches being orphaned from their directory. After clearing caches, the instances in memory don't have a file-system directory anymore. This means that either caching data would crash the app, or, in the case of baking a site, would make the workers create the directory but collide randomly while doing so at the same time.

*.pyc
venv/
build/lib
build/messages/_cache
dist/
piecrust.egg-info/
piecrust/__version__.py