Hi there !
I did a codingame in java recently and rapidly got a big file quite difficult to manage, so I did a small maven plugin that can take source files and package them all into a single file to be run on codingame.
It's quite basic but is working very well for me so I guess I could share it here. Here is the github repo:
The readme took me as much time to write as the code ... so it's not very clean but it does the job
So don't hesitate to try it if you feel the need