Feature request: Shave off dune-site specific things from the opam file generation

[kit-ty-kate]

Desired Behavior

It would be nice to only have the "--promote-install-files" "false" argument and subsequent dune install ... lines in the generated opam files only if dune-site is enabled.

Example

$ cat dune-project
(lang dune 3.0)
(using dune_site 0.1)
(generate_opam_files true)
[...]
$ cat pkg.opam
[...]
  build: [
    ["dune" "subst"] {dev}
    [
      "dune"
      "build"
      "-p"
      name
      "-j"
      jobs
      "--promote-install-files"
      "false"
      "@install"
      "@runtest" {with-test}
      "@doc" {with-doc}
    ]
    ["dune" "install" "-p" name "--create-install-files" name]
  ]
[...]

$ cat dune-project
(lang dune 3.0)
(generate_opam_files true)
[...]
$ cat pkg.opam
[...]
  build: [
    ["dune" "subst"] {dev}
    [
      "dune"
      "build"
      "-p"
      name
      "-j"
      jobs
      "@install"
      "@runtest" {with-test}
      "@doc" {with-doc}
    ]
  ]
[...]