Dispatching (a.k.a.) multiplexing services should be used in repo-agnostic services instead of local-repo-specific ones that have been used so far.
* services that return entities coming from a foreign repo, in particular implementations of EntityRevisionLookup, must apply the correct ID mapping for that repo during deserialization.
* for managing a set of service instances for a given repo, consider using a ServiceContainer. We should consider changing WikibaseClient to use core's new ServiceContainer mechanism alltogether.
* the ability to load e.g. labels for entities on another repo will be needed by repo code, not just client code. We will have to re-consider the relationship between repo and client once more.