Java FFI: enable inserting additionnal Java files to a project

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
Java FFI: javac is always called on local files

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
tests: add a test for the extra_java_files phase

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>