Can't resolve ppxlib.traverse when ppxlib is vendored

[gridbugs]

Expected Behavior

It should be possible to build a project which depends on the ppxlib.traverse library when the ppxlib package is vendored.

Actual Behavior

Compilation fails with the error:

  File "vendor/ppxlib.0.29.1/traverse/dune", line 9, characters 7-22:
  9 |   (pps ppxlib_metaquot)))
             ^^^^^^^^^^^^^^^
  Error: Library "ppxlib_metaquot" not found.

ppxlib_metaquot is part of the ppxlib package.

Reproduction

• PR with a reproducing test: Add repro for github issue 7543 #7544

Specifications

• Version of dune (output of dune --version): unreleased
• Version of ocaml (output of ocamlc --version): 4.14.1
• Operating system (distribution and version): NixOS

Additional information

This bug was introduced in 16a7e88. Relevant PR is #7415.

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

[anmonteiro]

fixed in #7553