New opam repository layout for large libraries

Introducing a new opam repository layout for managing large libraries and packages.

https://anil.recoil.org/notes/ocaml-opam-new-layout

Managing package manager constraints is getting difficult, particularly given the growth of the number of packages in the opam repository. I'm therefore laying out a new mechanism for the OCaml contributors to submit large package sets, such as those from Jane Street.