Generate (swh) releases from all git tags

The way we make snapshots of git repositories currently stores "lightweight tags" (that is, tags created without the git tag -a flag) as branches that point directly to revision objects.

This doesn't map properly to what git users mean when they do unannotated tags, so we should make sure that all git tags point to releases.

Note that this doesn't solve the question of pulling release notes from e.g. GitHub release pages, which is something that would need to be done by some other component (#17 comes to mind).

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Migrated from T2059 (view on Phabricator)