Move KotlinSourceSet back to org.jetbrains.kotlin.gradle.plugin.*

After all MPP related changes, the `KotlinSourceSet` interface turns out
compatible with the old `KotlinSourceSet`, and, since there are external
usages of `KotlinSourceSet`, it is reasonable to expose the new
interface under the old FQ-name to fix those external usages.

Issue #KT-26498 Fixed

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

@@ -9,7 +9,6 @@ import org.gradle.api.Named
 import org.gradle.api.attributes.HasAttributes
 import org.gradle.api.file.FileCollection
 import org.gradle.api.tasks.SourceSetOutput
-import org.jetbrains.kotlin.gradle.plugin.source.KotlinSourceSet
 
 interface KotlinCompilation: Named, HasAttributes, HasKotlinDependencies {
     val target: KotlinTarget

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

@@ -3,13 +3,11 @@
  * that can be found in the license/LICENSE.txt file.
  */
 
-package org.jetbrains.kotlin.gradle.plugin.source
+package org.jetbrains.kotlin.gradle.plugin
 
 import groovy.lang.Closure
 import org.gradle.api.Named
 import org.gradle.api.file.SourceDirectorySet
-import org.jetbrains.kotlin.gradle.plugin.HasKotlinDependencies
-import org.jetbrains.kotlin.gradle.plugin.LanguageSettingsBuilder
 
 interface KotlinSourceSet : Named, HasKotlinDependencies {
     val kotlin: SourceDirectorySet