As a stepping stone towards being able to update our intrinsic object identifiers, we should make sure our public API tags links with the "name" of the identifier used for cross-object links.

As an example, the "directory" endpoint shows some amount of that, but it's not entirely consistent:

https://archive.softwareheritage.org/api/1/directory/9f64905516d67e19fa53bc49ee33b765971a92ec/

Entries that point to contents show some checksums (not all), but still show an implicit "sha1_git" target without being explicit about which kind of checksum it is.