The upstream tends to maintain overlapping minor versions, so I think a
track is appropriate. I have seen them putting out simultaneous point
releases for 2.3.x, 3.0.x and 3.1.x for example, so I think we need to
be able to support that.
At some stage, we will want to be able to flag a track as obsolete, but
let's not worry too much about that for now.