JDK 11+ support for javac plugin indexer

[Strum355]

In Olaf's testing with graalvm JDK 11, there were compile issues with certain symbols from the com.sun.source packages not being resolved.
To support constructs beyond JDK 8 (records etc) we'll need to be able to compile with a newer JDK.

This was previously achieved in the Kotlin rewrite through compiling with runtime java version checks to avoid calling methods that don't exist in earlier versions.
Another approach we could try is with multi-release jars.

The issue of tools.jar not being a thing in JDK9+ is already taken care of by Olaf in the SBT plugin