antlr generated files written to wrong location on Windows

On the Windows platform, the antlr-generated java source files are written to the base output directory, rather than a sub-directory corresponding to the package name. They are written to the correct location on Mac and Linux.

To reproduce, run "gradle" on Windows. The antlr-generated java source files are in the base directory:

modules/javafx.graphics/build/gensrc/antlr/

They should instead be written to:

modules/javafx.graphics/build/gensrc/antlr/com/sun/scenario/effect/compiler/

This is not a regression and predates the antlr 4.7 upgrade. I discovered this bug while running the new package validation test implemented by JDK-8221993.

I plan to exclude the antlr-generated sources from the package validation test. They should be re-enabled as part of fixing this bug.