If we append, "weboob.x" will not be resolved by regular Finders so our
custom finder will be hit. However, for importing "weboob.x.y", standard
Finders will find something, basing on "weboob.x.__spec__", and eventually
duplicating python modules, precisely what we wanted to avoid.
To ensure our custom Finder is called even for "weboob.x.y", prepend it.