Will Thompson commented:
I do not think changing the DOAP files to set their homepage to the repo that holds the DOAP file is appropriate. Taking https://gitlab.gnome.org/GNOME/gnome-shell/blob/master/gnome-shell.doap as an example, it specifies:
<homepage rdf:resource="https://wiki.gnome.org/Projects/GnomeShell" />and that's true: https://wiki.gnome.org/Projects/GnomeShell is the homepage for Shell.
But take a look at https://github.com/gnome/gnome-shell – there is nothing “above the fold” which indicates that the repo is actually a mirror, and that the canonical repo is at https://gitlab.gnome.org/GNOME/gnome-shell. (Shell is unusual in that it actually does have several prominent links to that page in its README.md.)
I searched DuckDuckGo for gnome shell git and gave up looking for gitlab.gnome.org after 5 pages. The GitHub mirror and Endless and Pop_OS!'s forks are the top three hits for me, and GitLab even has a helpful popout box about the GitHub mirror.
My hypothesis is just that, if we have a prominent link to the canonical repo near the very top of each mirror page, humans will more easily find the GitLab repos. Mutter actually does both of the things in the ticket description:
My other hypothesis was that search engines are also more likely to rank the GitLab pages higher if the GitHub mirror links prominently to them. However, for Mutter, its canonical repo still isn't in the first 5 pages of DDG results; and on closer inspection, Google actually does get both of these repos right, with the GitLab.gnome.org page right at the top.
If you think that the GitHub "homepage" field must match the DOAP <homepage>, and if it's not possible to add the “mirror” tag + URL to repos like https://github.com/gnome/gnome-shell which don't already have it, then I guess this is WONTFIX :)