Refactored Typens into Typens & Prelude_utils
Prelude_utils is not dependent over Env nor any typing module, as a result, it
can be used in other context like ocamldep (see next commit)
First part of ocamldep update with namespaces
The prelude is elaborated into the old version of module aliases -> it does not
change too much of the code of the dependency computation (it does not have to
use the prelude AST). The module names environment has been updated as well, but
the dependencies are not correctly calculated yet.