feature: better spack repo management.

[bcumming]

Spack packages in uenv are defined in three different locations:

1. the builtin packages in spack/var/spack/repos/builtin
2. the repo path provided by stackinator
   • primarily for providing cray-mpich and it's dependencies, which are custom packages
3. the repo path in a recipe

Recently we have started puting packages in stackinator to provide up to date versions and fixes on Santis

• the latest cuda and nvhpc packages
• gcc with patches for Grace

There are two issues with this

1. if a recipe provides a package that is also provided by stackinator, there is a conflict during stack configuration.
2. by providing packages for an in-development system like Santis as part of stackinator, we enforce changes on all vClusters.

We can fix this by:

1. handle the case where multiple repos provide the same package with clear precidence
2. provide per-vCluster repos

I think the following precidence for packages from the different repos would be the best:

recipe > cluster > stackinator > spack