Do not wrap built-in function in JetLightClass (EA-38770)

Built-in top-level functions are located in jet.namespace class.
JetStandardClasses.isStandardClass() now checks that a given fqName starts with
"jet.", instead of searching it in the standard scope, to prevent this kind of
errors in the future

idea/testData/javaFacade/ea38770.kt

@@ -0,0 +1,3 @@
+fun <T> foo() {
+    T.<caret>toString()
+}