Add asb_utils_ensure_exists()
Add a dummy application for all packages

If --add-cache-id is specified and there are no file matches, we need to add a
fake application entry.
Create paths when setting up the AsbContext

It was pretty magic what was required, and this puts it all in one place.
Actually use the AsApp objects from the old metadata
Add self tests for re-using old metadata
Add as_store_set_builder_id()

This is used to identify builder versions with compatible metadata.
Do not write the builder-id in the cache-id