internal: Get rid of the whole "sub cache" business. * Compute cache keys up front, so the cache directory is only chosen once. * Buffer up config variants to apply before loading the config. Makes it possible to cache variant-resulting configs, too. * Make a factory class to reuse the logic that creates the `PieCrust` object correctly for multi-process workers and such. * Add a test.

# This is a utility module that can be used with any WSGI-compatible server
# like Werkzeug or Gunicorn. It returns a WSGI app for serving a PieCrust
# website located in the current working directory.
import os
from piecrust.wsgiutil import get_app


root_dir = os.getcwd()
app = get_app(root_dir)
# Add this for `mod_wsgi`.
application = app