twoliter: support for controlling package priority with kits

[jmt-lab]

As a customer I would like to be able to ensure that I can provide my own packages and prioritize selection of them in my variant builds over an overlap with bottlerocket defaults.

Example:

As a customer i may want to provide my own kubernetes-1.30 package and ensure that my package is selected and the bottlerocket-kubernetes-1.30 package is now. I also would like to be able to prioritize overlapping packages in multiple external kits should it occur.