Option to bootstrap/make_spack to include experiment recipe repos

[marcmengel]

The bootstrap/make_spack should subsume the script that mu2e is using to initialize Spack
by optionally including experiment recipe repository or repositories.

[marcmengel]

This at least includes optionally including https://github.com/Mu2e/mu2e-spack
and/or https://github.com/DUNE/dune_spack, etc.