[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.