Support generation and loading of layered jars

[philwebb]

As part of #19697 we need to update the Repackager to generate layered JARs and update the JarLauncher to support them.

[wilkinsona]

Re-opening as I think we need some equivalent Gradle changes and I don't want them to fall through the cracks.

[philwebb]

We'll handle the Gradle work in #19792

[philwebb]

I messed up and managed to not actually write the index file.