Extend the syntax of the key argument of "get" to support data packages

While introducing lb-nightly-functions!6 (merged) and trying to integrate data packages support in lb-nightly-functions!5 (merged) I realized that we do not have a way to identify data packages the same way we do for projects and slots.

Although we can run the checkout by asking for something like "slot/id/DBASE" and then loop over the packages (as we do at the moment), it probably make more sense to promote them and be able to have one checkout job per data package, with the plus of getting independent checkout logs for each data package.