Fix swh-docs's broken dev build

Build [1] has been broken for days, probably more.

WARNING: autodoc: failed to import module 'setup' from module 'swh.loader.cvs.rcsparse'; the following exception was raised:
No module named 'swh.loader.cvs.rcsparse.setup'; 'swh.loader.cvs.rcsparse' is not a package
/home/jenkins/workspace/DDOC/dev/swh-docs/docs/swh-loader.rst:6: WARNING: duplicated entry found in toctree: swh-loader-cvs/index
/home/jenkins/workspace/DDOC/dev/swh-docs/docs/apidoc/swh.loader.cvs.rcsparse.setup.rst: WARNING: document isn't included in any toctree
/home/jenkins/workspace/DDOC/dev/swh-docs/docs/swh-perfecthash/index.rst: WARNING: document isn't included in any toctree
/home/jenkins/workspace/DDOC/dev/swh-docs/docs/swh-vault/getting-started.rst:24: WARNING: undefined label: persistent-identifier

• [1] https://jenkins.softwareheritage.org/job/DDOC/job/dev/6791/console

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Migrated from T3648 (view on Phabricator)