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

[Arun Isaac <arunisaac@systemreboot.net>]

> We could maintain a cache of redirects, although that wouldn’t be a
> real solution.
>
> 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.  :-/

On the other hand, if we constrain our updater predicates to not perform
network operations, we would be restricting what we can achieve with
updaters. For example, suppose in the future we have a cgit-updater to
update packages hosted using cgit. The only way to detect that would be
using some kind of network operation. The way our updaters currently
work, we will only ever be able to properly handle centralized platforms
like GitHub, PyPI, etc. And, that's a pity.

Another solution, though somewhat inelegant and tedious, would be to add
an `updater' field to the package specification so that the packager can
explicitly specify what updater to use.

WDYT?