Modularise NixOS and home-manager configs

nixos/modules/_list.nix

@@ -0,0 +1,12 @@
+{
+  nixos.modules = {
+    common = ./common.nix;
+    user = ./user.nix;
+    build = ./build.nix;
+    dynamic-motd = ./dynamic-motd.nix;
+    tmproot = ./tmproot.nix;
+    firewall = ./firewall.nix;
+    server = ./server.nix;
+    deploy-rs = ./deploy-rs.nix;
+  };
+}

nixos/modules/common.nix

@@ -17,6 +17,11 @@ in
     documentation.nixos.options.warningsAreErrors = dummyOption;
   };
 
+  imports = [
+    inputs.impermanence.nixosModule
+    inputs.agenix.nixosModules.age
+  ];
+
   config = mkMerge [
     {
       home-manager = {