Request: Add capability to treat this as a spack repo.

[aaroncblack]

I would like to use the spack packages in this repo as a spack repo. Currently, to use these packages requires that I copy them into a spack installation and overwrite the packages in the builtin repo.

This becomes a maintenance headache. I would much prefer to bring this in as one of the repos of packages.

For example, if this repo is named 'llnl.radiuss'

repos:                                                                                                                                                                                                                                                                            
- $spack/../llnl.radiuss                                                                                                                                                                                                                                                      
- $spack/var/spack/repos/builtin rather do this:

I'll create a MR with the changes needed for review.