plugins/conform: change state semantics

Didn't realize `darwinOnly` didn't actually warn/fail install on linux
only. It just passes a state message to the auto-install function.
Change the state function to automatically handle the expected
condition.

plugins/by-name/conform-nvim/auto-install.nix

@@ -23,7 +23,12 @@ rec {
     name:
     let
       permittedNames = lib.optionals (lib.isAttrs configuredFormatters) (attrNames configuredFormatters);
-      stateList = map (state: lib.fix (lib.toFunction state)) (attrValues states);
+      stateList = lib.pipe states [
+        attrValues
+        (map lib.toFunction)
+        (map (fn: fn null))
+        (filter isString)
+      ];
       isState =
         maybePackage:
         lib.throwIf (isFunction maybePackage) "The '${name}' conform-nvim formatter package is a function" (

plugins/by-name/conform-nvim/formatter-packages.nix

@@ -6,7 +6,7 @@ with pkgs;
 let
   states = {
     broken = _package: "broken";
-    darwinOnly = _package: "Darwin only";
+    darwinOnly = p: if stdenv.hostPlatform.isDarwin then p else "Darwin only";
     unpackaged = "unpackaged";
   };
 in

tests/test-sources/plugins/by-name/conform-nvim/default.nix

@@ -11,7 +11,12 @@
         formatter-packages
         states
         ;
-      stateList = map (state: lib.fix (lib.toFunction state)) (builtins.attrValues states);
+      stateList = lib.pipe states [
+        builtins.attrValues
+        (map lib.toFunction)
+        (map (fn: fn null))
+        (builtins.filter builtins.isString)
+      ];
       allFormatters = builtins.filter (
         name:
         let