KT-4893 Code completion should not show multiple functions with the same signature

#KT-4893 Fixed

idea/idea-completion/testData/basic/multifile/PreferMoreSpecificExtension1/PreferMoreSpecificExtension1.dependency.kt

@@ -0,0 +1,4 @@
+package dependency
+
+fun Any.xxx(): Int = 1
+fun ppp.C.xxx(): Int = 1

idea/idea-completion/testData/basic/multifile/PreferMoreSpecificExtension1/PreferMoreSpecificExtension1.kt

@@ -0,0 +1,12 @@
+package ppp
+
+import dependency.*
+
+class C {
+    fun foo() {
+        xx<caret>
+    }
+}
+
+// EXIST: { lookupString: "xxx", itemText: "xxx", tailText: "() for C in dependency", typeText: "Int" }
+// NOTHING_ELSE