register sources on schema class, never instance

register sources on schema class, never instance

Change _invoke_loader in Loader.pm to always invoke the loader on the
Schema class, never the instance, even if an instance is passed in, and
then merge the state of the class into the instance if necessary.

Fix up a couple of tests to work with this new logic.

Also change make_schema_at to use ->connect instead of ->connection
because of people relying on instance state in ->connection (RT#74175),
being careful to copy the storage back to the Schema class.