Implement duck (Debian Url ChecKer) to report dead urls in d/control and d/upstream/metadata

duck.debian.net was a service that allowed users to check the validity of the urls in d/control and d/upstream/metadata.

With the previous maintainer going MIA, the service went down and because of a licensing issue, the site cannot be brought back as is.

As discussed previously on #963887, distro-tracker is a good candidate to re-implement this QA feature. This issue can be used to track down the process and technical choices.

The first step would be to write a small technical spec of how the checks will be done in distro-tracker.

I'm willing to do the work, although, I might not start right away.