Get rid of FileCollectionInternal usage that breaks with Gradle 5.0+

Remove the implementation of the `FileCollectionInternal` interface in
`KotlinCompilationOutput`. This interface may change unexpectedly (and
does between Gradle 4.10 and 5.0) so we need to get rid of its
implementation.

On the other hand, we can't just implement `FileCollection` instead
because Gradle internally assumes that all `FileCollection` instances
are also `FileCollectionInternal`. So the way to fix this is to remove
`SourceSetOutput` from superinterfaces of `KotlinCompilationOutput`
altogether.

Instead, to work with `SourceSetOutput` and `KotlinCompilationOutput`
in a uniform way, provide a wrapper for `SourceSetOutput`:
`KotlinWithJavaCompilationOutput` that is basically a proxy
class.

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

@@ -7,8 +7,15 @@ package org.jetbrains.kotlin.gradle.plugin
 
 import org.gradle.api.Named
 import org.gradle.api.attributes.HasAttributes
+import org.gradle.api.file.ConfigurableFileCollection
 import org.gradle.api.file.FileCollection
-import org.gradle.api.tasks.SourceSetOutput
+
+interface KotlinCompilationOutput {
+    var resourcesDir: Any
+    val classesDirs: ConfigurableFileCollection
+
+    val allOutputs: FileCollection
+}
 
 interface KotlinCompilation: Named, HasAttributes, HasKotlinDependencies {
     val target: KotlinTarget
@@ -21,7 +28,7 @@ interface KotlinCompilation: Named, HasAttributes, HasKotlinDependencies {
 
     var compileDependencyFiles: FileCollection
 
-    val output: SourceSetOutput
+    val output: KotlinCompilationOutput
 
     val platformType get() = target.platformType