All built-in plugins are TransformerPlugins. PostProcessingPlugin was supposed to be used by verifiers, processors etc. But there are not PostProcessorPlugins in internal plugins. Just like ResourcePrevisitor, PostProcessor could just be an internal interface for now. If we want to revisit Post processing plugins in future, we could add to (experimental) jlink API.

TransformerPlugin being the only remaining Plugin type, the visit method of TransformerPlugin can be moved to Plugin. And TransformerPlugin can be removed.