rename config.outputs to config.system.outputs

New rules: everything under "config" that isn't actually configuration
(e.g. build products) will in future live in config.system. This is
the first step.

default.nix

@@ -41,8 +41,8 @@ let
     ];
   }).config.system;
 in {
-  outputs = config.outputs // {
-    default = config.outputs.${config.hardware.defaultOutput};
+  outputs = config.system.outputs // {
+    default = config.system.outputs.${config.hardware.defaultOutput};
   };
 
   # this is just here as a convenience, so that we can get a