meta: Prefer self in default.nix

I originally wrote default.nix files to "import all files in a given
directory" however this turns out to not be necessary when flakifying a
configuration and defining all modules, overlays, and packages as
outputs.

Instead of making these "default.nix" files the source of truth for all
imports, it should be more useful to make the flake.nix the source of
truth and programmatically use attribute values as needed.

overlays/default.nix

@@ -1,8 +1,10 @@
+{ self, ... }:
+
 {
-  nixpkgs.overlays = [
-    (import ./base16-schemes.nix)
-    (import ./kitty.nix)
-    (import ./srb2.nix)
-    (import ./zola.nix)
+  nixpkgs.overlays = with self.overlays; [
+    base16-schemes
+    kitty
+    srb2
+    zola
   ];
 }