vim-utils: create less symlinks

For every vim plugin, we create a symlink to every file in the top
directory. It should be enough to create a symlink to the vim plugin
directly.

pkgs/misc/vim-plugins/vim-utils.nix

···
     '';
 
     linkVimlPlugin = plugin: packageName: dir: ''
-      mkdir -p $out/pack/${packageName}/${dir}/${lib.getName plugin}
-      ln -sf ${plugin}/${rtpPath}/* $out/pack/${packageName}/${dir}/${lib.getName plugin}
+      mkdir -p $out/pack/${packageName}/${dir}
+      ln -sf ${plugin}/${rtpPath} $out/pack/${packageName}/${dir}/${lib.getName plugin}
     '';
 
     link = pluginPath: if hasLuaModule pluginPath