Strong typing of swh.model.identifiers arguments

swh.model.identifiers is our set of core algorithms for identifier computation.

Unfortunately, these functions take dicts, and are very loosely typed. It would strengthen us a lot to use the swh.model.model objects instead.

To be able to do this, we should make sure that swh.model.identifiers is not being used outside of swh.model.

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Migrated from T2713 (view on Phabricator)