Replace direct usage of swh.model.identifiers with use of swh.model.model objects

So we can switch swh.model.identifiers to strong(er) typing.

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Migrated from T2715 (view on Phabricator)

marked this issue as related to #2713 (closed)

added Data Model priority:Low labels

mentioned in merge request swh-loader-mercurial!33 (closed)

mentioned in merge request !367 (merged)

closed with merge request !367 (merged)