Support multiarch and multistage builds

Major API iteration to:

• switch to multistage container builds for smaller containers
• instead of separate ocaml and opam containers, just generate
  a single opam one which can optionally have the system compiler
  or a locally compiled one.
• explicitly support aliases for distributions, and allow older
  distributions to be marked as deprecated.

Other changes:

• Update OPAM 2 build mechanism to use make cold.
• Drop support for opam1 containers; use an older library version for those.
• Also mark OCaml 4.05.0 and 4.06.0 as a mainline release for opam2 as well.