flakes: Migrate the config and miku to flakes

overlays/custom.nix

@@ -1,5 +0,0 @@
-pkgs: final: prev:
-
-import ../packages {
-  pkgs = prev;
-}

overlays/default.nix

@@ -3,21 +3,14 @@
 , lib
 }:
 
-let
-  importWrapper = file: import file {
-    pkgs = pkgs;
-    lib = lib;
-  };
-in {
+{
   emacs = import (builtins.fetchGit {
     url = "https://github.com/nix-community/emacs-overlay.git";
     ref = "master";
     rev = "ce0e9482d53d69bedc8416d8a984d00e17607826";
   });
   nur = import ./nur.nix pkgs;
-  unstable = importWrapper ./unstable.nix;
-  stable = importWrapper ./stable.nix;
-  custom = import ./custom.nix pkgs;
+  custom = import ../packages;
   simple = import ./simple.nix;
   surface = import ./surface.nix pkgs;
 }

overlays/stable.nix

@@ -1,21 +0,0 @@
-# NOTE: pkgs is unused but there for consistency with nur.nix
-{
-  lib, ...
-}: final: prev:
-
-let
-  stableTarball =
-    fetchTarball
-      https://github.com/NixOS/nixpkgs/archive/nixos-21.05.tar.gz;
-in {
-  stable = import stableTarball {
-    # TODO
-    config = {
-      allowUnfreePredicate = pkg: builtins.elem (lib.getName pkg) [
-        "firmwareLinuxNonfree" "microcodeAmd" "microcodeIntel"
-        "steam" "steam-original" "steam-runtime"
-        "discord"
-      ];
-    };
-  };
-}

overlays/unstable.nix

@@ -1,21 +0,0 @@
-# NOTE: pkgs is unused but there for consistency with nur.nix
-{
-  lib, ...
-}: final: prev:
-
-let
-  unstableTarball =
-    fetchTarball
-      https://github.com/NixOS/nixpkgs/archive/nixos-unstable.tar.gz;
-in {
-  unstable = import unstableTarball {
-    # TODO
-    config = {
-      allowUnfreePredicate = pkg: builtins.elem (lib.getName pkg) [
-        "firmwareLinuxNonfree" "microcodeAmd" "microcodeIntel"
-        "steam" "steam-original" "steam-runtime"
-        "discord"
-      ];
-    };
-  };
-}