In order to avoid having coqdep calls depend on all file contents, it would be useful to have a way to use coqdep like ocaml -modules.

This has 2 steps:

• make coqdep output the right stuff
• make dune use it

We need an output format for coqdep which can handle

• Declare ML Module "foo" "bar" (dependency on some cmxs)
• Require foo bar.baz and From x Require foo bar.baz (dependency on .vo)
• Load foo.bar and Load "foo" (dependency on .v)

coqdep also handles Add LoadPath but I don't think we want to support that.