Fix importlib usage

Fix importlib usage
We have to use as_file() on the final path, not the module path.

Because as_file() only learned to support directories in python 3.12,
we backport the 3.12 implementation temporarily in mkosi itself.

Because as_file() does not apply the executable bit, we apply it
ourselves after parsing the config. This requires delaying the check
if scripts are executable to some later point so we can parse the
config without failing because scripts are not executable.

This fixes mkosi zipapps.