Using `(root_module ...)` with a PPX results in a build failure

[craigfe]

Expected Behavior

The (root_module ...) feature should compose with (preprocess ...):

(executable
 (name main)
 (root_module root)
 (libraries unix)
 (preprocess (pps ppx)))

Actual Behavior

Combining (root_module ...) with a PPX results in a build failure:

  $ dune build
  Error: Multiple rules generated for _build/default/root.pp.ml-gen:
  - dune:5
  - <none>:1

Reproduction

• PR with a reproducing test: Add a reproduction case for issue #4682 #4683

Specifications

• Version of dune (output of dune --version): 2.8.5
• Version of ocaml (output of ocamlc --version): 4.12.0
• Operating system (distribution and version): Linux 5.12.6-arch1-1

/cc @rgrinberg @bobot

[bobot]

Indeed it should compasable. The preprocessing should not be applied to this file, in the same way than other alias files.