Feature/source mirrors

[grodzki-lanl]

Allows users to build from custom spack mirrors. Spack mirrors can be defined in recipes/config.yaml like so:

source_mirrors:
  <mirror1-name>: <url>
  <mirror2-name>: <url>

This will generate a site-scoped spack mirrors.yaml (via spack mirror add command in the Makefile) located in $(BUILD_ROOT)/spack/etc/spack

mirrors:
  mirror1-name: <url>
  mirror2-name: <url>
  spack-public:
    binary: false
    url: https://mirror.spack.io

The mirrors have precedence from top to bottom: spack will try to fetch from the first mirror, and will move down the list if the package is not found, defaulting to spack-public if none of the user-defined mirrors provide it.