Move to Spack Environments

[white238]

We need to move away from the old Spack usage of a split packages/compilers file (I don't think there is a word for it) and to Spack Environments.

I had a long conversation with @becker33 and this was his recommendation to convert:

# Translation of dev-build -u X in uberenv-speak
spack env create env /path/to/environment/file
spack -e env add $spec
# dev-build mode command
spack -e env develop -u final_phase--no-clone --path $location_of_axom $spec  # remove -u final_phase for install
# install and uberenv-pkg modes command
spack -e env install -u final_phase --no-add $spec  # remove -u final_phase for full install

Note this example is for dev-build with a final phase mode but has alterations for install and dev-build with no final phase.

[white238]

TODO:

• Move unified spack commands out of WIP spack envs support #75 into new PR, merge
• Have uberenv install host-config into uberenv prefix
• Create new Env PR and move current uberenv.py to pre_env_uberenv.py

Cyrus concerns (may not need to be fixed before):
#44
#46 (comment)