Load externals

[joeheaton]

Build env needs to detect externals

How do we have different externals on different systems with the same shared filesystem? cd into system-specific build dir?

Need to detect Spack "externals" after they are built. For example Python, otherwise a package requiring ^python3.8 will just build yet another python instead of use our python3.8... It's possible spack/spack#311 will improve this situation by making Spack re-use installed packages more aggressively.