Refactor vim plugins overlay

Add overlays for additions, modifications and unstable packages, and
move each additional plugin into its own additional package.

lib/nixos/configuration.nix

@@ -1,5 +1,6 @@
 {
   inputs,
+  outputs,
   desktop ? false,
   hostname,
   self,
@@ -18,7 +19,11 @@ in
       permittedInsecurePackages = [ "electron-27.3.11" ];
     };
 
-    overlays = [ (import "${self}/overlays/vim-plugins-overlay.nix") ];
+    overlays = [
+      outputs.overlays.additions
+      outputs.overlays.modifications
+      outputs.overlays.unstable-packages
+    ];
   };
 
   nix.nixPath = [ "nixpkgs=${inputs.nixpkgs}" ];

lib/nixos/default.nix

@@ -1,5 +1,6 @@
 {
   inputs,
+  outputs,
   nixos-hardware,
   pkgs,
   self,
@@ -12,6 +13,7 @@
 let
   configuration = import ./configuration.nix {
     inherit
+      outputs
       desktop
       hostname
       inputs

lib/wsl2/default.nix

@@ -1,5 +1,6 @@
 {
   inputs,
+  outputs,
   self,
   username,
 }:
@@ -9,7 +10,11 @@ let
   inherit (pkgs) lib;
 
   pkgs = import inputs.nixpkgs {
-    overlays = [ (import "${self}/overlays/vim-plugins-overlay.nix") ];
+    overlays = [
+      outputs.overlays.additions
+      outputs.overlays.modifications
+      outputs.overlays.unstable-packages
+    ];
   };
 
   shared-config = import "${self}/lib/shared/home-manager.nix" {