nix: Use the current merlin in 'nix develop'

The previous devShells installed 'ocaml-lsp', which depended on nixpkgs'
merlin. Since switching to OCaml 5.2 this no longer works but also makes
little sense.

This uses the overrideScope mechanism to make sure that anything added
to the devShells use the current merlin.

Unfortunately, 'ocaml-lsp' is not compatible with the current Merlin and
is dropped from the devShells.

flake.nix

@@ -21,10 +21,12 @@
               version = "20201216";
               src = menhir-repository;
             });
+
+            inherit (packages) merlin-lib dot-merlin-reader merlin;
           });
 
         inherit (ocamlPackages) buildDunePackage;
-      in rec {
+
         packages = rec {
           default = merlin;
           merlin-lib = buildDunePackage {
@@ -35,6 +37,7 @@
             propagatedBuildInputs = with ocamlPackages; [ csexp ];
             doCheck = true;
           };
+
           dot-merlin-reader = buildDunePackage {
             pname = "dot-merlin-reader";
             version = "dev";
@@ -44,6 +47,7 @@
             buildInputs = [ merlin-lib ];
             doCheck = true;
           };
+
           merlin = buildDunePackage {
             pname = "merlin";
             version = "dev";
@@ -69,9 +73,12 @@
             meta = with pkgs; { mainProgram = "ocamlmerlin"; };
           };
         };
+      in {
+        inherit packages;
+
         devShells.default = pkgs.mkShell {
           inputsFrom = pkgs.lib.attrValues packages;
-          buildInputs = with ocamlPackages; [ ocaml-lsp ];
+          buildInputs = with ocamlPackages; [ merlin ];
         };
       });
 }