launchpad: Reimplement lister using new Lister API

Port launchpad lister to the swh.lister.pattern.Lister API.

Last update date of each listed git repositories is now sent to the scheduler.

The lister can work in incremental mode, only modified repositories since the last listing operation will be returned in that case.

Closes #2992 (closed)

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Migrated from D4962 (view on Phabricator)