weboob compat: prepend our compatibility Finder instead of append

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.