Add missing __init__.py so find_packages keep finding sql modules

Otherwise, at some point, this will get discarded as per the debian build warning [1]

• [1] https://jenkins.softwareheritage.org/view/swh-debian%20(draft)/job/debian/job/packages/job/DSCH/job/gbp-buildpackage/182/console

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Migrated from D8952 (view on Phabricator)