Make swh-journal tests not depend on swh-model any more

Currently, swh-journal actually depends on swh-model only for the definition of KeyType, really. The model objects (BaseModel subclasses), are uses as an easy way to create dataset for the tests consisting in actual model objects. However nowadays, the journal writer depends on the ValueProtocol only.

This dependency on swh-model and on the actual model declared in swh.model.model is a burden because any modification in this model will (probably) break tests in swh-journal for no good reason.

The real dependency link we want to break is this later one, but then the only remaining dependency of swh-journal to swh-model is the definition of KeyType. I'd rather live with this definition duplicated in swh-journal than having a dependency.

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Migrated from T2970 (view on Phabricator)