* Classes for configurable plugins are a special case. They can either declare
* their configuration dependencies using the calculateDependencies() method
* described in the paragraph above, or if they have only static dependencies,
* these can be declared using the 'config_dependencies' annotation key.This is does not really tell the whole story. The configuration entity has to implement EntityWithPluginCollectionInterface and the plugins can implement ConfigurablePluginInterface. If the config entity implements EntityWithPluginCollectionInterface plugin providers and dependencies declared in the plugin's config_dependencies definition key (this does not have to be an annotation - it can be a derived definition) are added. If the plugin implements ConfigurablePluginInterface then each plugin instance's calculateDependencies() method is called.