new_package: create basic SRPM header as fallback
    
    Make SRPM handover between macros even more graceful and reliable by auto-creating
    a basic SRPM header before attempting declaration of a different sub-package.
    
    With this change things will just work as long as the default %{source_name}
    %{source_summary} and %{source_description} are set by something to sensible values.