Include agenix module and package directly

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

m/common/base/agenix.nix

@@ -1,9 +1,3 @@
-{ agenix, ... }:
-
 {
-  imports = [ agenix.nixosModules.default ];
-
-  environment.systemPackages = [
-    agenix.packages.x86_64-linux.default
-  ];
+  imports = [ ../../module/agenix.nix ];
 }