simplify overlay.nix

Move pkg definitions to pkgs/default.nix.
This allows us to just import the pkgs in overlay.nix and get rid of
the filtering to exclude the modules.

overlay.nix

@@ -1,21 +1 @@
-# You can use this file as a nixpkgs overlay.
-# It's useful in the case where you don't want to add the whole NUR namespace
-# to your configuration.
-
-self: super:
-
-let filterSet =
-      (f: g: s: builtins.listToAttrs
-        (map
-          (n: { name = n; value = builtins.getAttr n s; })
-          (builtins.filter
-            (n: f n && g (builtins.getAttr n s))
-            (builtins.attrNames s)
-          )
-        )
-      );
-in filterSet
-     (n: !(n=="lib"||n=="overlays"||n=="modules")) # filter out non-packages
-     (p: true) # all packages are ok
-     (import ./default.nix { pkgs = super; })
-
+self: super: import ./pkgs { pkgs = super; }