[WIP] Checkpoint command: check if an env needs to be re-concretized

[scheibelp]

> spack-python .../checkpoint.py update
(change a package)
> spack-python .../checkpoint.py check
unequal

For large environments, you can potentially save time concretizing using this command, which tells you when you might need to reconcretize.

It might be better implemented as a feature of environments (that is optionally enabled) vs. a spack-python script.

Examples of things that could affect a re-concretization (report unequal):

• Adding a spec
• Removing a spec
• Changing a spec in spack.yaml
• Changing a package.py file
• Adding a requirement
  • even for a package that is not part of the original concretization
• Adding a repository

Examples of things that would not affect a re-concretization (report equal):

• you changed the source code of a develop package
• you git pulled and no associated package.py changed
• you changed a package.py, but your env uses an alternative from another repo that does not inherit from it