URL glitch: double trailing slash needed to avoid 404 on deb package URLs

• this URL works: https://archive.softwareheritage.org/browse/origin/http://snapshot.debian.org/package/r-base//
• this gives 404: https://archive.softwareheritage.org/browse/origin/http://snapshot.debian.org/package/r-base/

The search results properly point to the working one, so it's not a very impactful issue for users coming from that entry point. But it is quite annoying for people trying to figure out browse URLs by hand-ish.

Not sure if there is a sure-fire way to fix this, but it's worth a try, as it'd improve URL friendliness.

(thanks @eddelbuettel for spotting this)

---

Migrated from T1852 (view on Phabricator)