[server] Deprecate Repository.sources_by_eid It's not used anymore within cubicweb itself.

[server] Add source_by_eid and source_by_uri methods to repository Most of the times we only need to retrieve one source (either by uri or eid) and querying sources_by_eid and sources_by_uri properties on repository just for one item is costly. So these methods query what's needed. We issue a ValueError (instead of KeyError for sources_by_{eid,uri} dict) in case the key is not found.

[hooks] Remove list() around repo.sources_by_uri There's no need to convert it as a list anymore since sources_by_uri is a property and will not be modified.

[server] Inline _entity_update method into init method of AbstractSource This _entity_update method does not make sense now that we do not update source from database information but always build them afresh.