Add extended Distro module

[benmandrew]

PR https://github.com/ocurrent/obuilder/pull/146 has grown into two separate parts: adding a Distro module that extends that of dockerfile-opam to include MacOS (and later BSD), and then extracting out shared spec file generation code from OCaml-CI and opam-repo-ci.

This is the reasoning for the strange name of the package, as PR https://github.com/ocurrent/obuilder/pull/146 will be rebased on top of this PR once it is merged.

(Note, this PR relies on a bugfix in dockerfile-opam being released - https://github.com/ocurrent/ocaml-dockerfile/pull/158.)