move flake-parts into pkgs or toplevel

pkgs/installer/flake-module.nix

@@ -0,0 +1,19 @@
+{ self, lib, ... }:
+let
+  installer = lib.nixosSystem {
+    pkgs = self.inputs.nixpkgs.legacyPackages.x86_64-linux;
+    system = "x86_64-linux";
+    modules = [
+      self.nixosModules.installer
+      self.nixosModules.hidden-ssh-announce
+      self.inputs.nixos-generators.nixosModules.all-formats
+      self.inputs.disko.nixosModules.disko
+      ({ config, ... }: { system.stateVersion = config.system.nixos.version; })
+    ];
+  };
+in
+{
+  flake.packages.x86_64-linux.install-iso = self.inputs.disko.lib.lib.makeDiskImage { nixosConfig = installer; };
+  flake.apps.x86_64-linux.install-vm.program = installer.config.formats.vm.outPath;
+  flake.apps.x86_64-linux.install-vm-nogui.program = installer.config.formats.vm-nogui.outPath;
+}