[test] Fix devtools unittest_i18n when ran without pytest
There is currently some weird error, requiring both to hack cubicweb.__path__
and to stop using load_module_from_name to load the registry.
Introduce a dependency on to-be-released version of logilab common integrating
fix for https://www.logilab.org/ticket/6085650
Do some extra import cleanups along the way.
