nixos manuals: bring back package references

This reverts most of 89e983786a, as those references are sanitized now.
Fixes #10039, at least most of it.

The `sane` case wasn't fixed, as it calls a *function* in pkgs to get
the default value.

nixos/modules/services/amqp/activemq/default.nix

@@ -32,6 +32,7 @@ in {
         '';
       };
       configurationDir = mkOption {
+        default = "${activemq}/conf";
         description = ''
           The base directory for ActiveMQ's configuration.
           By default, this directory is searched for a file named activemq.xml,
@@ -125,8 +126,6 @@ in {
       '';
     };
 
-    services.activemq.configurationDir = mkDefault "${activemq}/conf";
-
   };
 
 }