feat(flake): add module for overlays

modules/nixpkgs.nix

@@ -0,0 +1,60 @@
+{ config, lib, ... }:
+
+with lib;
+let
+  cfg = config.my.nixpkgs;
+
+  overlayType = mkOptionType {
+    name = "nixpkgs-overlay";
+    description = "nixpkgs overlay";
+    check = lib.isFunction;
+    merge = lib.mergeOneOption;
+  };
+in
+{
+  options.my.nixpkgs = {
+    overlays = mkOption {
+      default = [ ];
+      type = types.listOf overlayType;
+      example = literalExpression
+        ''
+          [
+            (self: super: {
+              openssh = super.openssh.override {
+                hpnSupport = true;
+                kerberos = self.libkrb5;
+              };
+            })
+          ]
+        '';
+    };
+    channels = mkOption {
+      default = { };
+      example = literalExpression ''
+        {
+          stable = inputs.nixpkgs-stable;
+        }
+      '';
+      type = with types; attrsOf package;
+    };
+    overlaysForAllChannels = mkEnableOption "apply overlays for all channels";
+  };
+
+  config.nixpkgs = {
+    overlays =
+      let
+        channelOverlays = _: prev:
+          mapAttrs
+            (_: value:
+              import value {
+                inherit (prev) system;
+                overlays = optional cfg.overlaysForAllChannels cfg.overlays;
+              }
+            )
+            cfg.channels;
+      in
+      cfg.overlays ++ [ channelOverlays ];
+
+    config.allowUnfree = true;
+  };
+}