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 
cheers