Do not remove import for iterator()

idea/testData/editor/optimizeImports/multifile/IteratorFunction/IteratorFunction_after.kt

@@ -0,0 +1,7 @@
+import test1.MyClass
+import test1.iterator
+
+fun foo() {
+    val s = MyClass()
+    for (i in s) {}
+}

idea/testData/editor/optimizeImports/multifile/IteratorFunction/main.kt

@@ -0,0 +1,7 @@
+import test1.MyClass
+import test1.iterator
+
+fun foo() {
+    val s = MyClass()
+    for (i in s) {}
+}

idea/testData/editor/optimizeImports/multifile/IteratorFunction/myClass.kt

@@ -0,0 +1,15 @@
+package test1
+
+public class MyClass {
+}
+
+public fun MyClass.iterator(): Iterator<MyClass> {
+     return object: Iterator<MyClass> {
+         override fun next(): MyClass {
+             throw Exception()
+         }
+         override fun hasNext(): Boolean {
+             throw Exception()
+         }
+     }
+ }