rename config/ -> cfg/

notably, this avoids the problem where `co<tab>` can't auto-complete the trailing slash.
`cf<tab>` autocompletes to `cfg/` in the same number of keystrokes :-)

configuration.nix

@@ -5,7 +5,7 @@
 { config, pkgs, ... }:
 
 {
-  imports = [ ./config ];
+  imports = [ ./cfg ];
 
   nixpkgs.overlays = [
     (self: super: {