replace 'trait' keyword with 'interface' in library code

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

@@ -6,7 +6,7 @@ import org.gradle.api.internal.file.FileResolver
 import org.gradle.api.internal.file.DefaultSourceDirectorySet
 import org.gradle.util.ConfigureUtil
 
-trait KotlinSourceSet {
+interface KotlinSourceSet {
 
     fun getKotlin(): SourceDirectorySet
     fun kotlin(configureClosure: Closure<Any?>?): KotlinSourceSet

libraries/tools/kotlin-gradle-plugin/src/test/resources/testProject/suppressWarningsAndVersion/src/helloWorld.kt

@@ -7,7 +7,7 @@ fun foo(p: Int??) {
 
 }
 
-trait T {
+interface T {
     abstract fun foo()
 }