Iterate Intention: Fix detection of extension iterators

#KT-8616 Fixed

idea/testData/intentions/iterateExpression/extensionIterator.kt

@@ -0,0 +1,8 @@
+// WITH_RUNTIME
+class T<U>
+
+operator fun <U> T<U>.iterator(): Iterator<U> = listOf<U>().iterator()
+
+fun test() {
+    T<Int>()<caret>
+}