ISPN-4761 Protobuf schema parser should not allow definitions that have a '.' separated name

* Add test for empty package name
* Also fix FileDescriptor.getFullName() to use '/' separator instead of '.'
ISPN-4758 Protobuf schema parser should not allow duplicate type definitions

* Also fix FieldDescriptor.setFileDescriptor() to not set a wrong FileDescriptor to types defined in another file
ISPN-4765 Protostream treats all imports as public