WikiPageEntityRevisionLookup loads entity data directly from a mediawiki database. It already supports accessign a "foreign" wiki database (not the local wiki). It now needs to support foreign entity IDs. To achieve this, we must do two things:
* check that the EntityId used to request the desired entity revision belongs to the correct repository. That is, EntityId::getRepositoryName should return the name of the repo to corresponds to the database name used by the WikiPageEntityRevisionLookup. Note that repo name and database name are //usually// but not necessarily the same. We can assume that the repo name is the same as the global site ID.
* the factory logic that instantiates the WikiPageEntityRevisionLookup must make sure to provide a EntityContentDataCodec that uses a Deserializer that is configured to apply the correct ID prefix mapping for the given foreign repo. We may want to introduce a ForeignEntityRevisionLookupFactory for this purpose, which can be used to acquire an EntityRevisionLookup for a given repo. Creating that interface should be tracked in a separate task.