Better support for OCaml dependencies in code using extension points

[foretspaisibles]

OCaml object files should depend on system libraries and 3rd party libraries, which would cause the expected recompilation when these libraries are upgraded.

[foretspaisibles]

The problem is caused by ocamldep not descending into expressions mentioning extension points. We should use ocamlfind ocamldep instead of ocamldep when appropriate.