fix: Add dune to existing packages to allow resolving

Signed-off-by: Marek Kubica <marek@tarides.com>

src/dune_pkg/opam_solver.ml

@@ -800,6 +800,13 @@ let reject_unreachable_packages =
           let version = Package_version.of_string "dev" in
           Some version)
     in
+    (* Dune is removed from the packages, but added here to allow formula
+       resolution to pick "dune" *)
+    let pkgs_by_version =
+      Package_name.Map.update pkgs_by_version Dune_dep.name ~f:(function
+        | Some _ as version -> version
+        | None -> Some (Package_version.of_string "dev"))
+    in
     let pkgs_by_name =
       Package_name.Map.merge pkgs_by_name local_packages ~f:(fun name lhs rhs ->
         match lhs, rhs with

test/blackbox-tests/test-cases/pkg/implicit-dune-constraint.t

@@ -38,6 +38,6 @@ Create a fake project and ensure `dune` can be used as a dependency:
   >  (allow_empty)
   >  (depends dune))
   > EOF
-  $ dune pkg lock 2>&1 | head -2
-  Internal error, please report upstream including the contents of _build/log.
-  Description:
+  $ dune pkg lock
+  Solution for dune.lock:
+  (no dependencies to lock)