Synthesize release objects for all upstream things that match the concept of a release

During the discussion about the branch name convention for the mercurial loader snapshots, in swh-loader-mercurial!3 (closed), @zack pointed out that the way we're avoiding synthesizing release objects for what our origins consider releases (for instance, lightweight git tags, PyPI/Debian packages, ...) is showing a two-fold problem:

it corrupts the fundamental Software Heritage data model for releases, by not tagging (heh) some objects that origins have marked as such
it therefore makes the UX confusing, by forcing our consumers to mix and match branch name conventions, origin-specific knowledge of the snapshot format, and the odd actual release objects to find software releases.

The consensus seems to be that we should

extend the release object data model to allow it to store arbitrary, intrinsic metadata
synthesize release objects for tarballs, Debian and PyPI packages
update the user interface to support the updated data model (most notably making sure direct release -> directory links can be resolved properly)

This meta-task will track the progress towards solving this issue

Migrated from T1258 (view on Phabricator)