Download correct JDiff binaries and sources to 'libx'

Download correct JDiff binaries and sources to 'libx'

This enables us to replace the original file from SCM. There is even an
improvement, like in other packages before: We create separate binary
and source archives, copying files from the compound download archive.
This way the library should be easy to use in an IDE.

TODO: This still does not get rid of the system path. Maybe it is better
to upload source and binary JARs to GitHub Packages.

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>