Adding a spack package repo instead of copying packages over to spack's builtin location

[chapman39]

In SpackEnv's patch function, we manually copy all of the packages in to spack's builtin package repo, but it would be better to get the location of the project's package repo (in something like --spack-package-repo-dir), and do spack repo add <spack_package_repo_dir> after creating the spack environment.

This would also involve creating a repo.yaml for the ci test project.

I was thinking about doing this after the environment's PR #96

https://spack.readthedocs.io/en/latest/repositories.html

[white238]

You can also add repos inside of Spack environment files, so this doesn't necessarily have to be an uberenv feature.