service: Adapt according to the latest storage.content_find changes

storage.content_find now returns List[Content].

This also adds types annotation to the impacted methods.

Related to T645

Related to swh-storage!989 (closed)

Test Plan

tox (failing until release > 0.11.9)

locally ok:

• pytest ok
• tox patched with swh-storage!989 (closed) ok ( .tox/py3/bin/pip install -e ../swh-storage

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Migrated from D3693 (view on Phabricator)