Define an architecture to fetch extrinsic metadata outside listers and loaders

Listers won't work to fetch metadata for all forges. On Github and on Bitbucket they don't get any metadata other than the description (and on Github, whether it's a fork). On GitLab they also get the number of stars and forks, but not much more.

Existing package loaders currently load some metadata, but we may want to use dedicated loaders for that, eg. with a speicifc visit type.

Or have a new kind of component outside listers and loaders (eg. if we want to use Github's endpoint to list an org's repos like etalab does)

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Migrated from T1739 (view on Phabricator)