vimPlugins.corePlugins: move to dedicated overlay file

pkgs/applications/editors/vim/plugins/corePlugins.nix

···
+{
+  symlinkJoin,
+}:
+final: prev: {
+
+  corePlugins = symlinkJoin {
+    name = "core-vim-plugins";
+    paths = with final; [
+      # plugin managers
+      lazy-nvim
+      mini-deps
+      packer-nvim
+      vim-plug
+
+      # core dependencies
+      plenary-nvim
+
+      # popular plugins
+      mini-nvim
+      nvim-cmp
+      nvim-lspconfig
+      nvim-treesitter
+      vim-airline
+      vim-fugitive
+      vim-surround
+    ];
+
+    meta.description = "Collection of popular vim plugins (for internal testing purposes)";
+  };
+
+}

pkgs/applications/editors/vim/plugins/default.nix

···
     inherit (neovimUtils) buildNeovimPlugin;
   };
 
+  corePlugins = callPackage ./corePlugins.nix { };
+
   # TL;DR
   # * Add your plugin to ./vim-plugin-names
   # * run ./update.py
···
   (extends luaPackagePlugins)
   (extends nodePackagePlugins)
   (extends nonGeneratedPlugins)
+  (extends corePlugins)
   (extends overrides)
   (extends aliases)
   lib.makeExtensible

pkgs/applications/editors/vim/plugins/overrides.nix

···
   fetchurl,
   neovimUtils,
   replaceVars,
-  symlinkJoin,
   # Language dependencies
   fetchYarnDeps,
   mkYarnModules,
···
   buildNeoVimPlugin = throw "New plugin definitions should be done outside `overrides.nix`";
 in
 {
-  corePlugins = symlinkJoin {
-    name = "core-vim-plugins";
-    paths = with self; [
-      # plugin managers
-      lazy-nvim
-      mini-deps
-      packer-nvim
-      vim-plug
-
-      # core dependencies
-      plenary-nvim
-
-      # popular plugins
-      mini-nvim
-      nvim-cmp
-      nvim-lspconfig
-      nvim-treesitter
-      vim-airline
-      vim-fugitive
-      vim-surround
-    ];
-
-    meta = {
-      description = "Collection of popular vim plugins (for internal testing purposes)";
-    };
-  };
-
-  #######################
-  # Regular overrides
-
   advanced-git-search-nvim = super.advanced-git-search-nvim.overrideAttrs {
     checkInputs = with self; [
       snacks-nvim