(transfo) UnitImporter is now a DfVisitor<Procedure>
(UnitImporter) Avoid name conflicts in variables/proc imported

All variables and procedure imported in an actor (duplicated in the
actor) are renamed. The new name is prepended with the Unit simpleName
(frontend) Check conflicts between vars, procs and functions names

For each function and procedure, the list of Actor/Unit variables,
functions and procedure are controlled to detect names conflicts
Fixes #1