- The setup-env.sh script currently makes two calls to spack, but it
should only need to make one.
- Add a fast-path shell setup routine in `main.py` to allow the shell
setup to happen in a single, fast call that doesn't load more than it
needs to.
- This simplifies setup code, as it has to eval what Spack prints
- TODO: consider eventually making the whole setup script the output of a
spack command