Take first satisfactory module found upstream, rather than latest.

This was inspired by a conversation with @hesco, who was one of the
first to tinker with multiple upstream repos.
A todo note for myself, to fix bug in the remote Install action.
PrerequisiteWalker should walk real Preqreq objects, not mere PackageSpec.
Update tests, now that cascading search is not the default.
Fix tests, following change from walking PackageSpec to Prereq objects.