During module definition, move conversion of packages from native to VM

During JDK 9 development I moved conversion of Strings from external to internal form into the JNI methods - see JDK-8171855 - which was an optimization.

Profiles show that we still have quite some overhead here though, partly due calling into VM from JNI. Which suggests moving the conversion completely into VM we can improve this even further.

An experiment show a reduction of around 1.2M instructions on bootstrap on a full image. While this ends up changing the API a lot, it also removes quite a bit of code.