writer: move the InMemoryJournalWriter from swh.storage in swh.journal
and put both JournalWriter implementations in a swh.journal.writer package. Also move the get_journal_writer() function from storage.
Keep a swh.journal.direct_writer module for bw compat.
This prevents a dependecy loop between swh.storage and swh.journal.
Migrated from D1992 (view on Phabricator)