[Gradle] Add dsl marker to KotlinSourceSet

We also need users to distinguish KotlinSourceSet DSL from other levels
of DSL, specifically extension one.

^KT-64722 In Progress

libraries/tools/kotlin-gradle-plugin-api/src/common/kotlin/org/jetbrains/kotlin/gradle/plugin/KotlinSourceSet.kt

@@ -8,8 +8,10 @@ package org.jetbrains.kotlin.gradle.plugin
 import org.gradle.api.Action
 import org.gradle.api.Named
 import org.gradle.api.file.SourceDirectorySet
+import org.jetbrains.kotlin.gradle.dsl.KotlinGradlePluginDsl
 import org.jetbrains.kotlin.tooling.core.HasMutableExtras
 
+@KotlinGradlePluginDsl
 interface KotlinSourceSet : Named, HasProject, HasMutableExtras, HasKotlinDependencies {
     val kotlin: SourceDirectorySet