[KAPT] Warn about usage of types from default package

Java doesn't support import from default package. 
We can't fix it, so we warn about it.
#KT-36743

plugins/kapt3/kapt3-cli/testData/integration/defaultPackage/Usage.kt

@@ -0,0 +1,7 @@
+package test
+
+import RootClass
+
+interface Usage {
+    fun test(): RootClass
+}