Optimize import removes reference to invoke extension function. Add test.

#KT-3398 Fixed

idea/testData/editor/optimizeImports/multifile/InvokeFunction/InvokeFunction_after.kt

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

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

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

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

@@ -0,0 +1,6 @@
+package test1
+
+public class MyClass {
+}
+
+public fun MyClass.invoke() {}