twoliter: support for controlling package priority with kits

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.

No files, tests, or entry points are named. Start by tracing how twoliter resolves packages from kits, then determine how variant builds should rank customer packages and overlapping packages from external kits. Done means the requested priority behavior is defined and covered for the Kubernetes example and cross-kit overlaps.

Written by the indexing model from the issue text.