cpan: Use a fake origin URL instead of an HTTP one

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 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.

Related to #2833

Depends on !323 (closed)

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Migrated from D8649 (view on Phabricator)