Write to trace in case class qualifier is a short reference to default object

This allows to fix some cases when there is a difference between explicit and short reference to default object
Fix shorten reference, optimize imports and import insert helper for default objects
ShortenReferences always transforms default object references to shorter form for now
Fix DescriptorUtils#getFqName() for default objects (affects test data mostly)
Fix DescriptorUtils#getImportableDescriptor()

compiler/testData/diagnostics/tests/imports/Imports.txt

@@ -81,8 +81,8 @@ package c {
 }
 
 package d {
-    internal val b: d.A.B
-    internal val c: d.A.B
+    internal val b: d.A.Default.B
+    internal val c: d.A.Default.B
 
     internal final class A {
         public constructor A()
@@ -103,7 +103,7 @@ package d {
                 public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
             }
 
-            internal object C : d.A.B {
+            internal object C : d.A.Default.B {
                 private constructor C()
                 public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
                 public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int