Use GitLab API as a secondary way of finding and fetching upstream releases.

Although it doesn't seem to be widely known, uscan permits listing multiple ways to check for upstream sources. This can defend against links getting broken or give uscan more strategies to try if it wants to do something a little uncommon, like if one asks to download a particular release instead of the latest version. GitLab is only used here to discover the links to source archives hosted at gimp.org since these are the "canonical" source archives. This maintains the status quo instead of using GitLab-generated archives that could be different.