Make inputs and inputs' available in pkgs arg

This allows functions with access only to pkgs to access the flake inputs. The
inputs' arg allows easy access to per system attributes.

default.nix

@@ -13,6 +13,11 @@ let
 
   baseModule = src: inputs: root: {
     withOverlay = final: prev: {
+      inherit inputs;
+      inputs' = mapAttrs
+        (_: mapAttrs
+          (_: v: v.${prev.system} or { }))
+        inputs;
       flakelite = exports // {
         meta = {
           platforms = root.systems;