minor refactoring in ocamldep.ml

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@15470 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
ocamldep support for -open

Adding an `open_module` function to the Depend interface instead of
directly mutating `free_structure_names` makes the approach robust to
eventual improvement to ocamldep's handling of `open` (for example, in
presence of .cmi for the opened units it could add their submodules
to the set of bound vars). This only goes so far, however, as the
overall imperative style would not nicely support proper local
handling of `let open M in e`.

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@15471 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
tell ocamlbuild about ocamldep's support of -open

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@15472 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02