[WIP] Add spack-python script to output diff in tree format

[scheibelp]

To use it, you run

spack-python altdiff.py spec1 spec2

e.g. for specs like:

$ spack spec --yaml dray+mpi > dray-mpi.yaml
$ spack spec --yaml dray~mpi > dray-nompi.yaml

Differences are highlighted red. There's quite a bit of extra output (and e.g. the exact same openmpi implementation appears in both DAGs because even in the dray~mpi case it ends up being used for a dependency), but the red highlighting is anticipated to be useful.

This command could also:

• separately list out the diffs to ensure they aren't missed
• optionally omit most details that are the same