add spack-repo-index.yaml for spack 1.0

[joequant]

BEGINRELEASENOTES

• add spack-repo-index.yaml which allows for git package loading in spack 1.0
  ENDRELEASENOTES

[joequant]

here is the release notes for spack 1.0

The two features that I really like are

• that they now have the packages in a separate git repository and you can reference external git repos in the spack
  config
• the compiler syntax is nicer
• concurrent package builds!!!!

https://github.com/spack/spack/releases#concurrent-package-builds

[tmadlener]

I suppose the more important change would be to add the package API to repo.yaml (as per the documentation)