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.

from piecrust.serving.server import WsgiServer


def get_app(root_dir, cache_key='prod', enable_debug_info=False):
    app = WsgiServer(root_dir,
                     cache_key=cache_key,
                     enable_debug_info=enable_debug_info)
    return app