Render FQ name of default package as "root package" in OverloadResolver

idea/testData/diagnosticMessage/conflictingOverloadsDefaultPackage.kt

@@ -0,0 +1,6 @@
+// !DIAGNOSTICS_NUMBER: 2
+// !DIAGNOSTICS: CONFLICTING_OVERLOADS
+// !MESSAGE_TYPE: TEXT
+
+fun foo(x: Int) = x
+fun foo(y: Int) = y

idea/testData/diagnosticMessage/conflictingOverloadsDefaultPackage1.txt

@@ -0,0 +1,2 @@
+<!-- conflictingOverloadsDefaultPackage1 -->
+'internal fun foo(x: kotlin.Int): kotlin.Int' is already defined in root package

idea/testData/diagnosticMessage/conflictingOverloadsDefaultPackage2.txt

@@ -0,0 +1,2 @@
+<!-- conflictingOverloadsDefaultPackage2 -->
+'internal fun foo(y: kotlin.Int): kotlin.Int' is already defined in root package