per_module breaks .merlin output #1212
Description
For the following dune file:
(library
(name Mercury)
(preprocess (per_module ((pps ppx_pgsql) Mercury)))
(modules Mercury)
(libraries pgocaml))
Merlin doesn't load the PPX for Mercury.ml. If I change the preprocess stanza to just (pps ppx_pgsql) then .merlin gains the required:
FLG -ppx '/home/dra/ocamldefang/_build/default/.ppx/ppx_pgsql/ppx.exe --as-ppx --cookie '\''library-name="Mercury"'\'''