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).

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Migrated from D3917 (view on Phabricator)