(minor) Move the unused source sets check to the taskGraph.whenReady

This is a workaround for Android compilations being configured in
an afterEvaluate callback.

Issue #KT-26963 Fixed

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

@@ -35,7 +35,8 @@ object UnusedSourceSetsChecker {
     }
 
     fun checkSourceSets(project: Project) {
-        project.afterEvaluate {
+        // TODO once Android compilations are configured eagerly, move this to afterEvaluate { ... } instead of taskGraph.whenReady { ... }
+        project.gradle.taskGraph.whenReady { _ ->
             val compilationsBySourceSet = compilationsBySourceSet(project)
             val unusedSourceSets = project.kotlinExtension.sourceSets.filter {
                 compilationsBySourceSet[it]?.isEmpty() ?: true