flake.nix: extract packages/devshells/modules into smaller files

nixosModules/testvm.nix

@@ -0,0 +1,34 @@
+{ virtwl }:
+{ pkgs, ... }: {
+  system.stateVersion = "25.11";
+
+  fonts.packages = [ pkgs.adwaita-fonts pkgs.dejavu_fonts ];
+  environment.systemPackages = [
+    pkgs.fastfetch
+    pkgs.htop
+    virtwl.packages.${pkgs.system}.proxy
+    pkgs.wayland-utils
+    pkgs.weston
+    pkgs.waycheck
+    pkgs.vulkan-tools
+    pkgs.glmark2
+    pkgs.mesa-demos
+    pkgs.xorg.xeyes
+    pkgs.xterm
+    # pkgs.vkquake # build broken: Program 'spirv-remap' not found
+    pkgs.veloren
+    pkgs.kdePackages.kate
+    pkgs.adwaita-icon-theme
+    pkgs.amberol
+    pkgs.bustle
+    pkgs.d-spy
+    pkgs.gnome-text-editor
+    pkgs.firefox
+    pkgs.ffmpeg-full
+    pkgs.mpv
+    pkgs.libva-utils
+    pkgs.tailscale
+    pkgs.zerotierone
+    pkgs.localsend
+  ];
+}