internal: Fix a severe bug with the file-system wrappers on OSX. `os.walk` is expected to support modifying its return values `dirnames` and `filenames` so that you can filter out items that need to be iterated upon. I was previously returning a different list in the wrappers, which means filtering was completely broken.

from piecrust.serving.server import Server


def get_app(root_dir, sub_cache_dir='prod', enable_debug_info=False):
    server = Server(root_dir,
                    sub_cache_dir=sub_cache_dir,
                    enable_debug_info=enable_debug_info)
    app = server.getWsgiApp()
    return app