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.

modules/squashfs.nix

@@ -10,8 +10,7 @@ let
 in
 {
   config = mkIf (config.rootfsType == "squashfs") {
-    outputs = rec {
-      rootfs = liminix.builders.squashfs config.filesystem.contents;
-    };
+    system.outputs.rootfs =
+      liminix.builders.squashfs config.filesystem.contents;
   };
 }