Rewrite celery tasks as a decorated function

as well as the tests of the task. Requires swh-scheduler >= 0.0.39

Bump dependency on swh-scheduler 0.0.39

Also use loader/mercurial as config base name for the sake of consistency (and to ease the docker env).

Related to T1480

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Migrated from D972 (view on Phabricator)

Build is green See https://jenkins.softwareheritage.org/job/DLDHG/job/tox/22/ for more details.

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approved this merge request

mentioned in issue swh/meta#1480 (closed)