Cleanup build and ensure kotlin sources are included in api JAR

Author: JLLeitschuh

build.gradle

@@ -442,7 +442,7 @@ subprojects { subproj ->
 
 		kotlin {
 			ktlint("0.9.0")
-			licenseHeaderFile headerText
+			licenseHeaderFile headerFile
 			trimTrailingWhitespace()
 			endWithNewline()
 		}

junit-jupiter-api/junit-jupiter-api.gradle

@@ -0,0 +1,41 @@
+import org.gradle.jvm.tasks.Jar
+import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
+
+dependencies {
+	"api"("org.opentest4j:opentest4j:${properties["ota4jVersion"]}")
+	"api"(project(":junit-platform-commons"))
+	compileOnly("org.jetbrains.kotlin:kotlin-stdlib")
+}
+
+tasks {
+	"jar"(Jar::class) {
+		manifest {
+			attributes(
+				mapOf(
+					"Automatic-Module-Name" to "org.junit.jupiter.api"
+				)
+			)
+		}
+	}
+}
+
+configurations {
+	"apiElements" {
+		/*
+		 * Needed to configure kotlin to work correctly with the "java-library" plugin.
+		 * See:
+		 * https://docs.gradle.org/current/userguide/java_library_plugin.html#sec:java_library_known_issues
+		 * https://youtrack.jetbrains.com/issue/KT-18497
+		 */
+		val compileKotlin: KotlinCompile by tasks
+		outgoing
+			.variants
+			.getByName("classes")
+			.artifact(mapOf(
+				"file" to compileKotlin.destinationDir,
+				"type" to "java-classes-directory",
+				"builtBy" to compileKotlin
+			))
+	}
+}
+

junit-jupiter-api/junit-jupiter-api.gradle.kts

@@ -21,5 +21,8 @@ include 'platform-tests'
 // based on the project name
 rootProject.children.each { project ->
 	project.buildFileName = "${project.name}.gradle"
+	if (!project.buildFile.isFile()) {
+		project.buildFileName = "${project.name}.gradle.kts"
+	}
 	assert project.buildFile.isFile()
 }