we already had this anyway, and it's already being reflected into.
Instead of DevTools checking for FolderPluginLoader instances, it
could just check if the file is a directory instead.
Plugin is the interface by which the server core interacts with plugins, so it should be limited only to the stuff that the server actually uses.
These methods are still provided by PluginBase, so in 99.9% of cases there will be no BC break.
