Merge: introduces `MGroup::default_mmodule`

There is no reason that "the module of a group that has the same name than the group and that is implicitly designated in importations when only the group's name is given" is not a concept of the model with a simple name name and a fully specified semantic.

A step toward #721 and maybe a help for #693

Pull-Request: #753
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>