redesign module hierarchy

nix/snow/flake/modules/overlays.nix

@@ -1,19 +1,18 @@
-{ lib, ... }:
-let
-  inherit (lib)
+{lib, ...}: let
+  inherit
+    (lib)
     mkOption
     types
     ;
-in
-{
+in {
   options = {
-    flake.overlays = mkOption {
+    outputs.overlays = mkOption {
       # uniq -> ordered: https://github.com/NixOS/nixpkgs/issues/147052
       # also update description when done
       type = types.lazyAttrsOf (types.uniq (types.functionTo (types.functionTo (types.lazyAttrsOf types.unspecified))));
       # This eta expansion exists for the sole purpose of making nix flake check happy.
       apply = lib.mapAttrs (_k: f: final: prev: f final prev);
-      default = { };
+      default = {};
       example = lib.literalExpression ''
         {
           default = final: prev: {};