LegacyIdInterpreter allows ItemId and PropertyId to be constructed from numeric IDs. We however need support for custom entities with numeric IDs too, at least as long as we do not have a replacement for the terms table (see T86530). EntityIdComposer provides an extensible mechanism for instantiating IDs for arbitrary entity types from numeric IDs. It should replace LegacyIdInterpreter everywhere.
This may require EntityIdComposer to be moved to WikiBaseServices.
LegacyIdInterpreter should be deleted when this is done.