Add a test to put a compilation unit in multiple containers
Remove duplicates in ocamldep outputs

(this could happen if both a ml and a mli were depending on the same modules)
Allow a compilation unit to belong to multiple containers (libs and bins)

There are still (limiter) mutation, as having a bi-directional link between containers and their contents is quite useful in practive. I'm not sure we can really get rid of the it completely.