deposit: Fix loader deposit inconsistency in revision committer date

The current deposit loader use the wrong date values for the committer date. This needs to be fixed to realign with the specification [5]

+-------------------+-----------------------------------------+----------------+----------------+
| SWH revision field| Description                             | CodeMeta term  | Fallback value |
+===================+=========================================+================+================+
| date              | date of software artifact modification  | dateCreated    | reception_date |
+-------------------+-----------------------------------------+----------------+----------------+
| committer_date    | date of the commit in VCS               | datePublished  | reception_date |
+-------------------+-----------------------------------------+----------------+----------------+

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Following revisions [1] [2] [3] which exposes or not issues.

In that regard, [1] is faulty because datePublished is provided so the committer date is wrong. Its author date is ok though.

• [2] is faulty because, the committer date should be the reception date. Its author date is ok though.

• [3] is ok though. As its the metadata fields provided are the same values (but that's plain luck ;)

• [1] https://archive.softwareheritage.org/api/1/revision/86e6c7e3ad0699601f73f343344a65f65b6f76ce/log/?fields=committer_date,date,metadata

• [2] https://archive.softwareheritage.org/api/1/revision/4631aacee153aab13d2c100c6716dfef5ec2a1ba/?fields=committer_date,date,metadata

• [3] https://archive.softwareheritage.org/api/1/revision/04a7cf3445bc390bc8dfaf93341a1f8f2008072c/?fields=committer_date,date,metadata

• [4] https://forge.softwareheritage.org/source/swh-loader-core/browse/master/swh/loader/package/deposit/loader.py$93-96

• [5] https://forge.softwareheritage.org/source/swh-deposit/browse/master/docs/specs/spec-loading.rst$217-219

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Migrated from T2368 (view on Phabricator)