backend_api: fix fetch_webc_package

[xdoardo]

Url::to_string() ends with a trailing slash, so this would have double slashes. We could just clone the URL and use Url::set_path(). But also, note that NamedPackageIdent has a registry field. But since this isn't actually actively used anywhere, we can do all of that in a followup to not block the merge.

Originally posted by @theduke in #4532 (comment)

[xdoardo]

Dupe of #4598. Closing.