mld files are installed

  $ dune build
  lib: [
    "_build/install/default/lib/foo/META"
    "_build/install/default/lib/foo/dune-package"
    "_build/install/default/lib/foo/opam"
  ]
  doc: [
    "_build/install/default/doc/foo/odoc-pages/doc.mld" {"odoc-pages/doc.mld"}
  ]
