With moving client into sc-service, we also made the light client features internal to sc-service. While this is fine for now, it means that you can't easily construct the equivalent of a light client without using the sc-service's ServiceBuilder anymore. However, only the new_light function in there is bound to client, everything else should be either in the respective crates (e.g. executor in sc-executor) or at least accessible without the need for sc-service. We should move service/client/light/{executor,backend,blockchain,fetcher} into a new crate sc-light to offer a single simple place to reuse the components.