Add ocamlfind packages

DO NOT MERGE

Just an experiment to infer ocamlfind packages by reading the contents of the `remove` field. Not sure OPAM 1.1 will be happy with this, so this PR is primary there to check. People keen to improve the inference (by looking inside the archive) are more than welcome. The relevant code is here: https://github.com/samoht/opam/blob/master/src/scripts/opam_libraries.ml