Extract objstorage from swh.storage module and recreate its commit history.
Summary: Extract swh.storage.objstorage to swh.objstorage and repare the outdated import to make the module work correctly.
Reviewers: olasd, zack, Reviewers, ardumont
Reviewed By: Reviewers, ardumont
Subscribers: ardumont
Maniphest Tasks: T460
Differential Revision: https://forge.softwareheritage.org/D63