Only the storage module needs it (because it's manipulating data as dict, which
i'm migrating away from in the intent to remove that proxy storage altogether
buta it's taking some time...)
Note: I moved the `swh_storage_backend_fixture` and `swh_storage` before the other
fixtures which use it (so the reading order is more natural).
This should fix the debian jenkins build [1]
[1] https://jenkins.softwareheritage.org/view/Debian%20packages/job/debian/job/packages/job/DLDBASE/job/gbp-buildpackage/153/console
^ the feedback loop for that is not good at all...