opam: Capture required libs as resources.

Despite `md5` being deprecated in favor of `sha1`, I use `md5` here
because the [`opam` Makefile uses MD5s][0] (and it seems sensible to
reflect that).

[0]: https://github.com/ocaml/opam/blob/4e90977926fa6993e0bc4bb4a4fbe6f0395f5e3f/src_ext/Makefile
opam: Adhere to zsh naming convention.
opam: Initialize the package DB in `post_install`

(rather than asking the user to do it)