nixos: create 'modules/nixos' folder

Let's consolidate all modules under one path, so that NixOS,
home-manager, and nix-darwin (if I ever end up using it down the line)
would go under the same folder.

modules/nixos/secrets/default.nix

@@ -0,0 +1,24 @@
+{ config, inputs, lib, ... }:
+
+{
+  imports = [
+    inputs.agenix.nixosModules.age
+  ];
+
+  config.age = {
+    secrets =
+      let
+        toName = lib.removeSuffix ".age";
+        userExists = u: builtins.hasAttr u config.users.users;
+        # Only set the user if it exists, to avoid warnings
+        userIfExists = u: if userExists u then u else "root";
+        toSecret = name: { owner ? "root", ... }: {
+          file = ./. + "/${name}";
+          owner = lib.mkDefault (userIfExists owner);
+        };
+        convertSecrets = n: v: lib.nameValuePair (toName n) (toSecret n v);
+        secrets = import ./secrets.nix;
+      in
+      lib.mapAttrs' convertSecrets secrets;
+  };
+}