KT-5697 Duplicate Unit type

#KT-5697 Fixed

compiler/testData/diagnostics/tests/multimodule/duplicateClass/collectionMethodStub.kt

@@ -0,0 +1,20 @@
+// FILE: f1.kt
+
+package kotlin
+
+class Unit
+
+// FILE: f2.kt
+
+class C: MutableIterator<Int> {
+    override fun remove(): Unit {
+        throw UnsupportedOperationException()
+    }
+    override fun next(): Int {
+        throw UnsupportedOperationException()
+    }
+    override fun hasNext(): Boolean {
+        throw UnsupportedOperationException()
+    }
+
+}