Do not allow to access compiler internals in .kts scripts

Set parent class loader to null explicitly instead of the system class
loader

compiler/testData/script/use_compiler_internals.kts

@@ -0,0 +1,14 @@
+fun checkInaccessible(name: String) {
+    try {
+        Class.forName(name)
+        throw AssertionError("Class should not be accessible from script via the class loader: $name")
+    }
+    catch (e: ClassNotFoundException) {
+        // OK
+    }
+}
+
+checkInaccessible("org.jetbrains.kotlin.config.KotlinCompilerVersion")
+checkInaccessible("org.jetbrains.kotlin.cli.jvm.K2JVMCompiler")
+checkInaccessible("org.jetbrains.kotlin.preloading.Preloader")
+print("OK")