Because PackageViewDescriptor may consist of several package fragments from
different modules (see LazyPackageViewDescriptorImpl#fragments), we now filter
out fragments from irrelevant modules before rendering them into the .txt
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()