- `spack env create <name>` works as before
- `spack env create <path>` now works as well -- environments can be
created in their own directories outside of Spack.
- `spack install` will look for a `spack.yaml` file in the current
directory, and will install the entire project from the environment
- The Environment class has been refactored so that it does not depend on
the internal Spack environment root; it just takes a path and operates
on an environment in that path (so internal and external envs are
handled the same)
- The named environment interface has been hoisted to the
spack.environment module level.
- env.yaml is now spack.yaml in all places. It was easier to go with one
name for these files than to try to handle logic for both env.yaml and
spack.yaml.