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

[scheibelp]

Description

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.

(Update Dec. 17 2024) you can pass --truncate in order to omit most details that match, like:

spack-python lib/scripts/altdiff.py --truncate ./dray-mpi.yaml ./dray-nompi.yaml 

This command could also:

• separately list out the diffs to ensure they aren't missed
• optionally omit most details that are the same
  • Update Dec 17 2024: --truncate option added

Dependencies: FIXME:Add a list of any dependencies.

Fixes issue(s): FIXME:Add list of relevant issues.

Type of Change

• { } Adding a system, benchmark, or experiment
• { } Modifying an existing system, benchmark, or experiment
• { } Documentation update
• { } Build/CI update
• { } Benchpark core functionality

Checklist:

If adding/modifying a system:

• { } Create a new directory for the system and a new system.py file
• { } Add a new dry run unit test in .github/workflows
• { } System appears in System Specifications table in docs catalogue section

If adding/modifying a benchpark:

• { } Add a new application.py and (maybe) package.py under a new directory
  for this benchmark
• { } Configure an experiment
• { } Benchmark appears in Benchmarks and Experiments table in docs catalogue
  section

If adding/modifying a experiment:

• { } Extend experiment.py under existing directory for specific benchmark
• { } Define a single node and multi-node experiments

If adding/modifying core functionality:

• { } Update docs
• { } Update .github/workflows and .gitlab/ci unit tests (if needed)