The autoloader won't load files in the folder `assets` as already stated it is all explained [here][1]. Your asset bundles should be in a folder called `resources` otherwise Yii won't register them in its classmap automatically. 

Your namespaces must represent your folder structure. The namespace `ns\prefix\controller` must be for a file in `vendor\ns\prefix\src\controller` so your namespace `ns\prefix\assets` will search for a file in `...vendor\ns\prefix\src\assets` but you have to include the resource on your own via [require_once][2] since Yii won't so it because you are in the wrong folder 

The easiest solution would be to create a folder `resources` and use the namespace `ns\prefix\src\resources`

Or you can - like stated in the docs - use your plugin root path as well `recoveryfinder\entriesbygroup`

  [1]: https://docs.craftcms.com/v3/asset-bundles.html
  [2]: http://php.net/manual/de/function.require-once.php