swh.core.tarball: Add support for more tarballs format

This should allow a better branch coverage in the package loaders (nixguix for one [1]).

• [1] swh/meta$734

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Migrated from T2623 (view on Phabricator)

added Core & foundations priority:Normal labels

It's been done for a while now. And there is a mechanism in place in swh.core.tarball to extend it further if need be. Closing.

assigned to @ardumont

closed