metadata-only: Restrict swhid use of qualifier anchor and visit on snapshot

I don't know what such SWHID would mean but technically it's a valid one (as far as i understood the grammar [1])

Nonetheless, we currently have a technical restriction when writing extrinsic metadata though. We can only have one snapshot reference possible there...

Let's discuss what to do then ;)

In this diff, I currently propose to log and return a bad request for such case.

That's the gist of the discussion, is that reasonable? If not, what to do instead?

• [1] https://docs.softwareheritage.org/devel/swh-model/persistent-identifiers.html#syntax

Related to #2537 (closed) !342 (closed)

Test Plan

tox

---

Migrated from D4493 (view on Phabricator)