Improve error location on import_scope

This improves errors "unknown scope" which are now more precise and target the qualid instead of the whole encapsulated function.

Also fix Loc.join: when b1 < a2, (Loc.join [a1; b1] [a2; b2]) = [a1; b2]

An example of improved errors is the following:

A.B.C (e := 3;
       f := 5) 

When A, B, or C is not found only the qualification is highlighted A.B.C instead of the whole program including the assignments.