wrong call to --ppx flags (slash issue)

Expected Behavior

We should be able to execute test of ppx_import

Actual Behavior

We can't compile test

Reproduction

• PR with a reproducing test: not yet

1. clone
2. dune runtest

Specifications

• Version of dune (output of dune --version): https://github.com/ocaml/dune/commit/ef1ee9a0d175bb076d0debf917f7c67a3939af6a
• Version of ocaml (output of ocamlc --version): 4.13
• Operating system (distribution and version): windows

Additional information

I tried to debug:
This line seem the culprit https://github.com/ocaml/dune/blob/78fa5d22c3cc09345bfe3eb25fb57e0cd7e3c4bc/src/dune_rules/preprocessing.ml#L293

The command is: ocamldep.opt.exe -modules -ppx ".ppx/57e0400f3b219ff4fc65d1fbd820e35d/ppx.exe --as-ppx" -impl src_test/ppx_deriving_sexp/test_ppx_deriving_sexp.ml

Since -ppx go through Sys.command. So we need to not normalize:
ocamldep.opt.exe -modules -ppx ".ppx\57e0400f3b219ff4fc65d1fbd820e35d\ppx.exe --as-ppx" -impl src_test/ppx_deriving_sexp/test_ppx_deriving_sexp.ml work as intended.

I tried to replace with: Path.Build.L.relative build_dir (".ppx" :: key :: "ppx.exe" :: []) but Path module seem to like normalized path /:
https://github.com/ocaml/dune/blob/ef1ee9a0d175bb076d0debf917f7c67a3939af6a/otherlibs/stdune/path.ml#L253

Have you any pointer to continue ?

• Link to gist with verbose output (run dune with the --verbose flag):

Start with src/dune_rules/preprocessing.ml around line 293 and compare its path handling with otherlibs/stdune/path.ml around line 253. Run dune runtest for the ppx_import reproduction on Windows and inspect the generated ocamldep command. Done means the ppx test compiles and the command uses a Windows-compatible path without breaking other platforms.