Controlling precedence of data_only_dirs vs vendored_dirs

• The current stage of dune about directory status is:

  You can't have data_only_dirs and vendored_dirs at the same time and that is logic.

  > cat dune
  (vendored_dirs subproject)
  (data_only_dirs subproject)
  
  > dune build 
  Error: Directory subproject was marked as data_only and vendored, it can't be
  marked as both.
  

  But you can have (dirs :standard \ subproject) combine with (data_only_dirs subproject) or (vendored_dirs subproject).

  The status (data_only_dirs subproject) or (vendored_dirs subproject) override (dirs :standard \ subproject), why this specification ?

  And what is weird to me is (dirs :standard \ subproject) is equivalent to (data_only_dirs subproject).

• The need we have is at the moment is:
  A possibility, (dirs :standard \ subproject) override data_only_dirs and/or vendored_dirs instead inverse. This avoids the conflict(ocaml-dockerfile) described below.

  An example: this is a partial tree of solver-service

  > tree solver-service
  solver-service
      |_ dune
      |_ dune-project
      |_ ocaml-dockerfile
          |_ dune-project
      |_ ocaml-ci
          |_ dune
          |_ dune-project
          |_ ocaml-dockerfile
               |_ dune-project
       
  > cat solver-service/dune
  (vendored_dirs ocaml-dockerfile) ...
  > cat soler-service/ocaml-ci/dune
  (vendored_dirs ocaml-dockerfile) ...
  
  > cd solver-service
  > dune build
  Error: Too many opam files for package "dockerfile-opam":
  - ocaml-ci/ocaml-dockerfile/dockerfile-opam.opam
  - ocaml-dockerfile/dockerfile-opam.opam
  

• The central point of this PR is to introduce or improve the spec around statuses:

  Getting the prioritization as following (dirs :standard \ subproject) -> (data_only_dirs subproject) -> (vendored_dirs subproject) .

  It is opened to modification, to get the possibility that (dirs :standard \ subproject) override data_only_dirs and/or vendored_dirs without touching the previous relation between data_only_dirs and vendored_dirs.

Originally posted by @moyodiallo in https://github.com/ocaml/dune/issues/6577#issuecomment-1514553706

No implementation files or tests are named. Start by reproducing the two dune build examples from issue #7589, then trace how dirs, data_only_dirs, and vendored_dirs statuses are specified and prioritized. Done means the precedence is explicitly specified and the nested ocaml-dockerfile case no longer produces the conflicting package-discovery result.