Impossible to import a file from a .jar

[CoolGuyMC]

It is impossible to import a file from a .jar when running a plugin on source.
For example, EconomyAPI:

import me.onebone.economyapi.EconomyAPI;

Won't work if i'm running the plugin on source, but when i run it on a .jar, it works fine.

[fengberd]

This might need to modify code of nukkit to fix it...

[XDqrkneZ]

^