We stop depending on the ListerBase implementation. The main hoop we're jumping
through is the config override mechanism in swh.lister.get_lister, as it's
really specifc to the ListerBase override_config argument, which is dropped in
pattern.Lister (in favor of overriding a load_config method).
This generic configuration override mechanism will probably be completely
superseded when all lister instances are reimplemented. We'll see.
Depends on D3425.