Rejects stanzas in subdirectories of `(include_subdirs unqualified)`.

  $ dune build
  File "src/gen/dune", line 1, characters 0-23:
  1 | (executable (name gen))
      ^^^^^^^^^^^^^^^^^^^^^^^
  Error: This stanza is not allowed in a sub-directory of directory with
  (include_subdirs unqualified).
  Hint: add (include_subdirs no) to this file.
  [1]
