map download link to distname

[Jonathan Stickel]

I'm working on updating the graphviz portfile. The new download link is:

https://gitlab.com/graphviz/graphviz/-/package_files/8183714/download

I can get Macports to download it by using:

master_sites  https://gitlab.com/${name}/${name}/-/package_files/8183714
distfiles     download

But the download source file is named "download". How do I rename it to 
the proper source file name "graphviz-2.47.0.tar.gz"? Forgive me if it 
is obvious, but I couldn't find this information in the documentation or 
an example in another Portfile.

Thanks,
Jonathan