Introduce new API

These interfaces are needed for both new MPP design and getting rid of
the Java plugin source sets in favor of Kotlin source sets

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

@@ -1,26 +0,0 @@
-/*
- * Copyright 2010-2016 JetBrains s.r.o.
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- * http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-package org.jetbrains.kotlin.gradle.plugin
-
-import groovy.lang.Closure
-import org.gradle.api.file.SourceDirectorySet
-
-interface KotlinSourceSet {
-    val kotlin: SourceDirectorySet
-
-    fun kotlin(configureClosure: Closure<Any?>?): KotlinSourceSet
-}