OPAM depext plugin

Replacement for the currently used shell-scripts handling distribution-specific installation of OPAM packages' external dependencies (as per the depexts field in their definitions).

This is a first prototype, largely incomplete ; it may help pave the way to some specification of the depexts field -- and may give some basis for a real plugin integrating with OPAM (see https://github.com/ocaml/opam/blob/master/doc/design/depexts-plugins)