[RFC] Allow context initialization to use `ocaml` binaries from workspace

It would be very cool if we could compose the OCaml compiler itself in a Dune workspace (c.f. https://github.com/ocaml/ocaml/issues/10315 ). A key step is to actually teach Dune to also look for OCaml binaries produced by rules in the current workspace.

This is subtle, thus I'm making an issue to discuss possible approaches; as of today, the code initializing the context does:

    let* ocamlc =
      get_tool_using_findlib_config "ocamlc" >>= function
      | Some x -> Memo.Build.return x
      | None -> (
        which "ocamlc" >>| function
        | Some x -> x
        | None -> prog_not_found_in_path "ocamlc")
    in
    let dir = Path.parent_exn ocamlc in
    let get_ocaml_tool prog =
      get_tool_using_findlib_config prog >>| function
      | Some x -> Ok x
      | None -> (
        match Program.best_path dir prog with
        | Some p -> Ok p
        | None ->
          let hint =
            sprintf "ocamlc found in %s, but %s/%s doesn't exist (context: %s)"
              (Path.to_string dir) (Path.to_string dir) prog
              (Context_name.to_string name)
          in
          Error
            (Action.Prog.Not_found.create ~context:name ~program:prog ~loc:None
               ~hint ()))

thus, findlib is tried first, if not, path is tried (IIUC)

I could image two ways to tweak this:

• make ocaml binary resolution just use the regular mechanism from Super_context.resolve_program
• add yet a first special case for the resolution on the current workspace

Good news is that all the tools are already Action.Prog.t in Context.t , except for ocaml which is a path, I wonder why.

Start with the context-initialization code shown in the issue, then read Super_context.resolve_program and the definitions of Context.t and Action.Prog.t. Compare the existing findlib and PATH lookup with workspace rule resolution. Done means agreeing on an approach and covering resolution of OCaml binaries produced in the current workspace.