Allow JavaPlugins to inject classes from ScalaPlugins into their own ClassLoaders.
This extra method (ScalaPluginLoader#openUpToJavaPlugin(ScalaPlugin,JavaPlugin) injects the classes from scala plugins directly into the PluginClassLoader of a JavaPlugin, preventing it from searching the 'global' classes cache of the JavaPluginLoader.
This method does not inject the classes from scala libraries, just the classes from the ScalaPlugins themselves.