Add package name override support via 'test-package-override' directive

Adding option to override package name for discover_build step. For example for nodejs in RHEL10 it would not find anything by the name 'nodejs' (yet). I would like to set my own name to lookup builds.

Sort of workaround for https://issues.redhat.com/browse/OSCI-8413, because now we will be able to proceed with generate_tier0 step with any package we provide in the description. And take control later on in plan.fmf if we wish to change what package to pull etc.