Kapt: Register kapt classes output directory as Java classes directory (KT-19211)

This is relevant to pure (non-Android) Java projects and Gradle 4.0+.

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/internal/Kapt3KotlinGradleSubplugin.kt

@@ -110,6 +110,7 @@ class Kapt3KotlinGradleSubplugin : KotlinGradleSubplugin<KotlinCompile> {
             val javaCompile: AbstractCompile,
             val kaptVariantData: KaptVariantData<*>?,
             val sourceSetName: String,
+            val javaSourceSet: SourceSet?,
             val kaptExtension: KaptExtension,
             val kaptClasspath: MutableList<File>) {
         val sourcesOutputDir = getKaptGeneratedDir(project, sourceSetName)
@@ -168,7 +169,7 @@ class Kapt3KotlinGradleSubplugin : KotlinGradleSubplugin<KotlinCompile> {
         val kaptExtension = project.extensions.getByType(KaptExtension::class.java)
 
         val context = Kapt3SubpluginContext(project, kotlinCompile, javaCompile,
-                kaptVariantData, sourceSetName, kaptExtension, kaptClasspath)
+                kaptVariantData, sourceSetName, javaSourceSet, kaptExtension, kaptClasspath)
 
         val kaptGenerateStubsTask = context.createKaptGenerateStubsTask()
         val kaptTask = context.createKaptKotlinTask()
@@ -283,6 +284,10 @@ class Kapt3KotlinGradleSubplugin : KotlinGradleSubplugin<KotlinCompile> {
         kaptTask.destinationDir = sourcesOutputDir
         kaptTask.classesDir = classesOutputDir
 
+        javaSourceSet?.output?.apply {
+            tryAddClassesDir { project.files(classesOutputDir).builtBy(kaptTask) }
+        }
+
         kotlinCompile.source(sourcesOutputDir, kotlinSourcesOutputDir)
 
         if (kaptVariantData != null) {

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

@@ -193,7 +193,7 @@ internal class Kotlin2JvmSourceSetProcessor(
     }
 }
 
-private fun SourceSetOutput.tryAddClassesDir(
+internal fun SourceSetOutput.tryAddClassesDir(
         classesDirProvider: () -> FileCollection
 ): Boolean {
     val getClassesDirs = ReflectionUtil.findMethod(