Fix some flake8 errors

[sources] Turn classmethod into standard method No need for a class method and easier to get proper logger including source's name.

[test] Add some tests for hooks.syncsources for parts which are not located in some source's check_config / check_urls method.

[sources] Check source's url attribute value on creation/modification Similarly as for config. Now, ldap source validation is properly done there instead of at initialization time.

[sources] Check sources configuration is fine on creation/modification Reintroduce usage of 'source.check_config' which had almost disappeared, as well as tests for the currently detected errors. Part of the system source specific checking done in syncsources reimplemented in a specific check_config implementation. Tests are dispatched among ldap / datafeed and syncsources tests but are not strictly correctly located (notably syncsources tests behaviour of the native source's check_config). The system source url checking part which disappears from syncsources will be reintroduced in a follow-up.

[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.

[server] Drop update_config method of source It does not make sense anymore to update the config of a source instance (subclass of cubicweb.server.sources.AbstractSource) now that they are always built from database information (CWSource). In datafeed and ldapfeed, we move all code from "update_config" method in "init" method. This changeset fixes LDAPFeedUserDeletionTC.test_a_filter_inactivate() failure (unittest_ldapsource.py) introduces in previous changeset.