Expose 'allKotlinSourceSets' in KotlinCompilation (KT-28749)

As KotlinCompilation only contains the directly added source sets in its
`kotlinSourceSets` property, we need to provide a convenient way to
collect the whole source sets hierarchy for a compilation.

We added internal `allKotlinSourceSets` some time ago, and it seems
reasonable to expose it as public API.

Issue #KT-28749 Fixed

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

@@ -31,6 +31,8 @@ interface KotlinCompilation<out T : KotlinCommonOptions> : Named, HasAttributes,
 
     val kotlinSourceSets: Set<KotlinSourceSet>
 
+    val allKotlinSourceSets: Set<KotlinSourceSet>
+
     val defaultSourceSet: KotlinSourceSet
 
     fun defaultSourceSet(configure: KotlinSourceSet.() -> Unit)