[test] Fix option name in ldap wrong group test which wasn't actually testing what it expected (and nothing guarantee it does now, but that's another story). Fixing this will avoid failure once we properly check the configuration.

[sources] Enhance prototype of check_conf_dict * mark it private * explicit name * update docstring Private function is still called from syncsources but this will be updated in a follow-up.

[sources] Stop translating validation error this should be done later in the web ui, where lang is properly set.

[sources] Simplify source's init method Only call it when enabled instead of giving a boolean flag indicating whether it is or not (which were not correctly considered).

[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.