Take ambiguity into account when using -short-paths option.
    Namely, do not use an abbreviated form which doesn't correspond
    directly to a physical definition when it might point to
    several definitions, due to multiple opens.
    Also be careful about not abbreviating types we are defining
    in type definitions.
    
    
    git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13635 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02