CPAN hosts a lot of legacy modules known as backpan that do not have
an HTML landing page so use fake origin URL pattern below instead:
`cpan://{author}/{module_name}`
author corresponds to the normalized CPAN user account, not the full
author name, while module_name is the distribution name.
For instance the distribution [File-ManualFlock](https://fastapi.metacpan.org/v1/release/_search?q=distribution:File-ManualFlock) is a backpan
so URL https://metacpan.org/dist/File-ManualFlock does not exist and returns 404.
The only HTML page we can found for this distribution is the backpan directory for the associated CPAN user [WCATLAN](https://cpan.metacpan.org/authors/id/W/WC/WCATLAN/).
Related to T2833
Depends on D8648