Use absolute import
cli: speedup the swh
cli command startup time
by moving import statements in functions and stop importing swh.objstorage.factory.* in swh.objstorage.
Warning: this later breaks a few dependencies that had not updated the way they import get_objstorage.
Related to swh-environment#2575 (closed).
Migrated from D3917 (view on Phabricator)