unify overlays

We used to have overlay definitions in home/overlays and
packages/default. Let's unify them.
Using the exposed overlay from the flake required some changes in the
home-manager config though.

flake.nix

@@ -109,6 +109,7 @@
           # Optionally use extraSpecialArgs
           # to pass through arguments to home.nix
           extraSpecialArgs = {
+            # XXX: can we use the inputInjection pattern here as well instead?
             inherit inputs;
           };
         };