Implement support for non-kts scripts in gradle

#KT-26784 fixed

build-common/src/org/jetbrains/kotlin/build/sourcesUtils.kt

@@ -0,0 +1,9 @@
+/*
+ * Copyright 2010-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
+ * that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.build
+
+val DEFAULT_KOTLIN_SOURCE_FILES_EXTENSIONS = listOf("kt", "kts")
+

build-common/src/org/jetbrains/kotlin/incremental/fileUtils.kt

@@ -21,11 +21,8 @@ import java.io.File
 fun File.isJavaFile() =
         extension.equals("java", ignoreCase = true)
 
-fun File.isKotlinFile(): Boolean =
-    extension.let {
-        "kt".equals(it, ignoreCase = true) ||
-        "kts".equals(it, ignoreCase = true)
-    }
+fun File.isKotlinFile(sourceFilesExtensions: List<String>): Boolean =
+    !isJavaFile() && sourceFilesExtensions.any { it.equals(extension, ignoreCase = true) }
 
 fun File.isClassFile(): Boolean =
         extension.equals("class", ignoreCase = true)