vimPlugins: generate an overlay

let update.py generates generated.nix as an overlay.
This should help maintaining vim-plugins outside of nixpkgs, as one can
use update.py to generate an out-of-tree vimPlugins overlay.

pkgs/misc/vim-plugins/default.nix

@@ -5,8 +5,12 @@ let
 
   inherit (vimUtils.override {inherit vim;}) buildVimPluginFrom2Nix;
 
+  inherit (lib) extends;
+
+  initialPackages = self: {};
+
   plugins = callPackage ./generated.nix {
-    inherit buildVimPluginFrom2Nix overrides;
+    inherit buildVimPluginFrom2Nix;
   };
 
   # TL;DR
@@ -21,8 +25,13 @@ let
     inherit llvmPackages;
   };
 
-  aliases = lib.optionalAttrs (config.allowAliases or true) (import ./aliases.nix lib plugins);
+  aliases = if (config.allowAliases or true) then final: prev: {} else (import ./aliases.nix lib);
 
+  extensible-self = lib.makeExtensible
+    (extends aliases
+      (extends overrides
+        (extends plugins initialPackages)
+      )
+    );
 in
-
-plugins // aliases
+  extensible-self