use swh.model.merkle/from_disk instead of swh.scanner.model

as per title

potential outcomes: (1) swh.model.merkle completely addresses swh.scanner.model needs (hence we remove the latter), (2) s.m.merkle needs some generalization, (3) s.s.model needs to use s.m.merkle + some additional status

either way, we want to refactor the duplicate logic

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Migrated from T3349 (view on Phabricator)