fix(inputs'): do not unnecessarily evaluate formatters

This compromises the ability to `inputs'?formatter`, but that seems
like a particularly rare use case.
Instead, we treat `inputs'` as another "programmatic" as opposed to
"polished" interfaces, similar to the `config` vs flake attributes
distinction.
The programmatic interfaces always prioritize laziness whereas the
polished interfaces may choose to omit known-missing things, to a
reasonable degree.

modules/formatter.nix

@@ -100,10 +100,9 @@ in
               '')
           config.allSystems);
 
-    perInput = system: flake:
-      optionalAttrs (flake?formatter.${system}) {
-        formatter = flake.formatter.${system};
-      };
+    perInput = system: flake: {
+      formatter = flake.formatter.${system};
+    };
 
   };
 }