ocamlobjinfo usage in dune

[last-genius]

I'm trying to see if I could pick up #650. It's fairly simple to do this as an external binary - just parsing and comparing the output of, say, dune describe external-lib-deps and ocamlobjinfo FILE, but I'm trying to see if I could expand the types in describe_external_lib_deps to also include a used boolean parameter.

One complication - ocamlobjinfo has a comment noting its parse function should only be used in tests (it is - in ocamlobjinfo_tests). Is there any other way to get information on which libraries were actually linked into the final binary? Thanks!

Note: Looking through git history (5fed01556d492ee42a4db7d9acc12e394d3664e1, for example), I can see dune used to parse ocamlobjinfo's output more extensively, but it's hard to track what happened with this since.