[bug#33801] import: github: Support source URIs that redirect to GitHub

[Arun Isaac]

> Hmm, now that I think about it, shouldn’t we store the github.com URL
> directly for these packages?  We could add a new lint check along the
> lines of the ‘check-mirror-url’ procedure that would allow us to find
> out which URLs need to be changed.  If we took that route, the changes
> you made to the importer would no longer be necessary.  :-/

My changes only took a small amount of effort. I don't have much of an
issue with them being made obsolete. Shall I proceed with creating a
lint check along the lines of what you proposed?