Add Kotlin SourceDirectorySet as extension

KotlinSourceSet now should be accessed via Kotlin project extension.
Conventions are still available, but not used inside plugins.

^KT-47047 In Progress

kotlin-native/Interop/Indexer/build.gradle.kts

@@ -142,9 +142,6 @@ sourceSets {
             srcDirs("prebuilt/nativeInteropStubs/kotlin")
         }
         kotlin{
-            target {
-
-            }
         }
     }
 }