[K2] A lot of ERROR_CLASS during building Gradle API reference

[whyoleg]

Describe the bug
There are a lot of ERROR_CLASS in DSL sourceSet. Probably there is some wrong with configuration of sources which worked for K1 but doesn't work for K2.

Screenshots
K1:

K2:

To Reproduce

1. Clone https://github.com/whyoleg/gradle/tree/dokka-gradle-plugin-v2 (branch dokka-gradle-plugin-v2)
2. Run ./gradlew :docs:dokkaGeneratePublicationHtml
3. Check output in platforms/documentation/docs/build/dokka/html

K1/K2 can be switched in https://github.com/whyoleg/gradle/blob/dokka-gradle-plugin-v2/platforms/documentation/docs/gradle.properties

[whyoleg]

This issue was caused by wrong configuration of custom source-sets: with K2 analysis we need to wire source-sets to have resolve work for DSL source-set
Fixed in whyoleg/gradle@041c5bd.