Added LaunchpadLister

First draft for (#1734 (closed))

It uses only launchpadlib and no http requests. This is because we've been discussing with the people at launchpad and they're planning on adding a new endpoint git_repositories.getAllRepositories just for this. Using only the library it will be easier to implement once they're done.

It is functionnal but tests are not ready yet. Looking for improvements and remarks on its design as it overrides almost everything.

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Migrated from D2799 (view on Phabricator)