java: use single javac invocation per jar

Instead of invoking javac for every .java file, pass all of the sources
for a jar target to a single javac invocation. This massively improves
first compilation time and doesn't meaningfully affect incremental builds
(it can even be faster in some cases).

The old approach also had issues where files would not always get recompiled
even though they should, necessitating a clean rebuild in order to see changes
reflected in the build output.

Multiple invocations seem to only make sense if:
- issues with files not getting flagged for rebuild are investigated and fixed
- something like the javaserver buildtool from openjdk sources is used
instead of directly spawning javac processes
- the amount of java files per jar is so large that it is faster to compile
several files one by one than to compile all the files at once (batching may
still make sense to get a reasonable balance)

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