Move default.nix into overlays dir

This is to prepare for splitting the overlay into two seperated ones.

Author: jian-lin

flake.nix

@@ -10,7 +10,7 @@
     }: {
       # self: super: must be named final: prev: for `nix flake check` to be happy
       overlay = final: prev:
-        import ./default.nix final prev;
+        import ./overlays final prev;
     } // flake-utils.lib.eachDefaultSystem (system: (
       let
         pkgs = import nixpkgs {