First step of implementation
For now, using namespaces is an AST transformation: the "with Mod of Ns" is
elaborated into "module Mod = Ns_mod". Shadowed modules and wilcard are not yet
implemented since it is the second step: the ability to look in the directories
for modules. Shadowed modules needs the wildcard (otherwise they are useless),
and wildcard must look for the correct directories in the loadpath.