languages/haskell.nix: Address PR comments

- Move formatting responsibility to conform-nvim, instead of HLS
- Wrap formatter binaries separately now that we are not using the
HLS-bundled libraries
- Add helpful assertion message to enforce the "not HLS if
haskell-tools" invariant.
- Add helpful assertion message to enforce the "not DAP if not
haskell-tools" invariant.

modules/plugins/lsp/presets/haskell-language-server.nix

@@ -15,14 +15,27 @@ in {
   };
 
   config = mkIf cfg.enable {
-    vim.extraPackages = [pkgs.haskellPackages.cabal-fmt];
     vim.lsp.servers.haskell-language-server = {
       enable = true;
-      cmd = [(getExe' pkgs.haskellPackages.haskell-language-server "haskell-language-server-wrapper") "--lsp"];
+      cmd = [
+        (getExe' (pkgs.symlinkJoin {
+          name = "haskell-language-server-wrapper";
+          paths = [pkgs.haskellPackages.haskell-language-server];
+          meta.mainProgram = "haskell-language-server-wrapper";
+          buildInputs = [pkgs.makeBinaryWrapper];
+          postBuild = ''
+            wrapProgram $out/bin/haskell-language-server-wrapper \
+              --prefix PATH : ${pkgs.haskellPackages.cabal-fmt}/bin
+          '';
+        }) "haskell-language-server-wrapper")
+        "--lsp"
+      ];
       root_markers = ["hie.yaml" "stack.yaml" "cabal.project" "*.cabal" "package.yaml"];
       settings = {
         haskell = {
-          formattingProvider = "ormolu";
+          # formatting is handled by conform-nvim; disable HLS's built-in formatter
+          formattingProvider = "none";
+          # cabal-fmt is an external tool; it is wrapped into the LSP binary's PATH above
           cabalFormattingProvider = "cabal-fmt";
         };
       };