Issue #2184951 by not_chx: Allow plugins to declare themselves a derivative.

...mponent/Plugin/Discovery/DerivativeDiscoveryDecorator.php

core/modules/comment/comment.views.inc

...omment/lib/Drupal/comment/Plugin/views/row/CommentRow.php

...modules/node/lib/Drupal/node/Plugin/views/row/NodeRow.php

core/modules/node/node.views.inc

.../system/lib/Drupal/system/Tests/Plugin/PluginTestBase.php

...n_test/lib/Drupal/plugin_test/Plugin/MockBlockManager.php

...st/Plugin/plugin_test/mock_block/MockMenuBlockDeriver.php

...modules/user/lib/Drupal/user/Plugin/views/row/UserRow.php

core/modules/user/user.views.inc