Package loader: Uniformize author and message
Since we are going to switch to releases, this looks like the right time to make some changes in the objects written by package loaders to make them consistent with each other.
Authors: use the empty string '' instead of placeholders Message: use the same message format (inspired by the Debian loader) for all loaders, instead of the empty string / the version / something else; except for PyPI and Deposit (which have a better format because we have more metadata available).
Additionally, this commit adds test of each release object, instead of only relying on its hash.
Depends on !239 (closed), !419 (closed), swh-model!203 (closed), swh-storage!745 (closed), swh-lister!247 (closed)
Test Plan
Tests will fail because this depends on diffs in swh-listers, swh-model, and swh-storage (in addition to other diffs in the same repo)
Migrated from D6629 (view on Phabricator)