pkg: dune with `pkg enabled` fails to build an oxcaml project without a lock

[shonfeder]

Expected Behavior

If a dune-workspace is properly configured with (pkg enabled) and to use oxcaml, then we should be able to install and build oxcaml using dune package management, without fixing a lock (i.e., without running dune pkg lock).

Actual Behavior

Build fails with a patch file not found.

Reproduction

#12850

Specifications

• Version of dune (output of dune --version): trunk

• Version of ocaml (output of ocamlc --version): n/a

• Operating system (distribution and version):

  System Version: macOS 15.7.2 (24G325)
  Kernel Version: Darwin 24.6.0
  

[Sudha247]

This also happens with dev tools, for example, when I try to install ocamllsp, it's not able to find the patches that are needed.