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/secret2.age

@@ -0,0 +1,5 @@
+age-encryption.org/v1
+-> ssh-ed25519 V3XmEA OB4+1FbPhQ3r6iGksM7peWX5it8NClpXIq/o5nnP7GA
+FmHVUj+A5i5+bDFgySQskmlvynnosJiWUTJmBRiNA9I
+--- tP+3mFVtd7ogVu1Lkboh55zoi5a77Ht08Uc/QuIviv4
+<EFBFBD><EFBFBD>X<EFBFBD>{<7B><>O<EFBFBD><4F><1F><04>tMXx<58>vӪ(<28>I<EFBFBD>myP<79><50><EFBFBD><EFBFBD>+3<>S3i