source.py: Track marked URLs and assert they are marked during Plugin.configure()

This cannot test for unaliased URLs, as those can be discovered later
on outside of user provided element configuration; at least we
assert that if an alias was used, we have seen it at load time.

This will cause a BUG to occur for a plugin which marks an aliased
URL (or attempts to translate one) outside of `Plugin.configure()`,
if that URL was not previously seen.

This is a part of #620