Gradle 6.0 removes source set dependency

[emign (Nico)]

Hi,
I have a collection of lessons for students here: https://github.com/emign/engineEmi_Lektionen/tree/dev_Stoppuhren

The dependant lib for this project gets injected via a gradle plugin here:
https://github.com/emign/engineEmi_GradlePlugin/blob/98a70b6a54c21c730a9d1cb6e4fee9ac369b8fc6/src/main/kotlin/me/emig/engineEmi/gradle/EngineEmiGradlePlugin.kt#L43

It all works great up until gradle 5.6.4. But when I upgrade the wrapper, it breaks and loses the Source Set of the above mentioned library. Can anyone help me with that?

Steps to reproduce:

1. Clone repo
2. Use gradle wrapper version 5.6.4 -> Example works. (Source set dependency engineEmi is integrated)
3. Use gradle wrapper version 6.0 or above -> Example breaks (Source set dependency engineEmi is missing)

[emign (Nico)]

I added some screenshots for better elaboration

[emign (Nico)]