Implement source sets visibility inference via associate compilations

Add non-public API for the IDE to query additional visible source sets
for each source set.
Implement visibility inference via associate compilation links.
Implement visibility requirements for source sets and requirements
satisfaction checks in Gradle build.

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

@@ -42,5 +42,8 @@ interface KotlinSourceSet : Named, HasKotlinDependencies {
 
     val customSourceFilesExtensions: Iterable<String> // lazy iterable expected
 
+    val requiresVisibilityOf: Set<KotlinSourceSet>
+    fun requiresVisibilityOf(other: KotlinSourceSet)
+
     fun addCustomSourceFilesExtensions(extensions: List<String>) {}
 }