Allow altering facets processor plugins through hook

Patch still applies for 2.x. In most cases users have control about the processor configuration. I had an edge-case where I wanted to manipulate the HierarchyProcessor, which is not that easy to do. For that the alteration of plugin info is quite helpful.

Thanks for the work. I need it for HierarchyProcessor too :D .
I add a usage example as reference:

/**
 * Implements hook_facets_processor_info_alter().
 */
function custom_module_facets_processor_info_alter(array &$info) {
  $info["hierarchy_processor"]["class"] = 'Drupal\custom_module\Plugin\facets\processor\HierarchyProcessor';
}

I don't think that this is a good idea.
In general you could simply add a new processor instead of replacing the class. And the new one could inherit from existing ones.

The Hierarchy processor was special. But it has plugins now.