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)

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/model/builder/KotlinModelBuilder.kt

@@ -23,7 +23,6 @@ import org.jetbrains.kotlin.gradle.plugin.KOTLIN_JS_DSL_NAME
 import org.jetbrains.kotlin.gradle.plugin.KotlinSourceSet
 import org.jetbrains.kotlin.gradle.plugin.getConvention
 import org.jetbrains.kotlin.gradle.plugin.mpp.KotlinAndroidTarget
-import org.jetbrains.kotlin.gradle.plugin.mpp.allKotlinSourceSets
 import org.jetbrains.kotlin.gradle.tasks.AbstractKotlinCompile
 
 /**

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/mpp/kotlinCompilations.kt

@@ -61,6 +61,9 @@ abstract class AbstractKotlinCompilation<T : KotlinCommonOptions>(
 
     override val kotlinSourceSets: MutableSet<KotlinSourceSet> = mutableSetOf()
 
+    override val allKotlinSourceSets: Set<KotlinSourceSet>
+        get() = kotlinSourceSets.flatMapTo(mutableSetOf()) { it.getSourceSetHierarchy() }
+
     override val defaultSourceSet: KotlinSourceSet
         get() = target.project.kotlinExtension.sourceSets.getByName(defaultSourceSetName)
 
@@ -178,9 +181,6 @@ abstract class AbstractKotlinCompilation<T : KotlinCommonOptions>(
     override fun toString(): String = "compilation '$compilationName' ($target)"
 }
 
-val KotlinCompilation<*>.allKotlinSourceSets: Set<KotlinSourceSet>
-    get() = kotlinSourceSets.flatMapTo(mutableSetOf()) { it.getSourceSetHierarchy() }
-
 abstract class AbstractKotlinCompilationToRunnableFiles<T : KotlinCommonOptions>(
     target: KotlinTarget,
     name: String