cargo-maelstrom: We produce warning when packages requires features not enabled

[bobbobbio]

Right now if there is some package that doesn't get built due to it requiring some feature that isn't enabled. We say something like "didn't receive build notification for PackageId { ... }".

On one hand notifying the user that there were package whose tests we didn't run sounds like a good idea, but on the other hand it being a warning is maybe undesirable. Maybe we should just somehow make it part of the summary, or else just kill the warning.