Include agenix module and package directly

Avoids adding an extra flake input only to fetch a single module and
package.

Reviewed-by: Aleix Boné <abonerib@bsc.es>
Tested-by: Rodrigo Arias Mallo <rodrigo.arias@bsc.es>

pkgs/agenix/example/secrets.nix

@@ -0,0 +1,23 @@
+let
+  user1 = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIL0idNvgGiucWgup/mP78zyC23uFjYq0evcWdjGQUaBH";
+  system1 = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIPJDyIr/FSz1cJdcoW69R+NrWzwGK/+3gJpqD1t8L2zE";
+in
+{
+  "secret1.age".publicKeys = [
+    user1
+    system1
+  ];
+  "secret2.age".publicKeys = [ user1 ];
+  "passwordfile-user1.age".publicKeys = [
+    user1
+    system1
+  ];
+  "-leading-hyphen-filename.age".publicKeys = [
+    user1
+    system1
+  ];
+  "armored-secret.age" = {
+    publicKeys = [ user1 ];
+    armor = true;
+  };
+}