Common klib support in the Kotlin Gradle plugin (KT-32677)

Refactor the Kotlin/Native compilations and tasks to support
Kotlin/Native-shared source sets compilation within the metadata target.

Provide the feature flag kotlin.mpp.enableCommonKlibs that enables
the following behavior:
* compilation of Kotlin/Native-shared source sets to a klib using
the Kotlin/Native compiler.
* compilation of common source sets (not just Kotlin/Native) into a
klib rather than *.kotlin_metadata

Issue #KT-32677 Fixed

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

@@ -33,6 +33,8 @@ interface KotlinCompilation<out T : KotlinCommonOptions> : Named, HasAttributes,
 
     val allKotlinSourceSets: Set<KotlinSourceSet>
 
+    val defaultSourceSetName: String
+
     val defaultSourceSet: KotlinSourceSet
 
     fun defaultSourceSet(configure: KotlinSourceSet.() -> Unit)