Support for fallback revisions

[pbsds]

Sometimes a script may know it needs version 1.2.3, but it doesn't know whether the upstream repo prepends v on their version tags or not. In such cases a list of fallback revisions to try would be great. Example

nurl repo-url v1.2.3 1.2.3

would try repo-url/v1.2.3 first, if it doesn't exit then try repo-url/1.2.3.

Especially pessimistic scripts could then even fallback on main or master.