In addition to the configuration scopes introduced in #2152 we also need a project scope.

Typical use case... I'm building two software stacks with different version/dependency selections in them. Or I want to run an integration test with a packages.yaml different from my "normal" one. Suggested ways to find the project scope would be:

1. An env var
2. A command line argument. (This would have to be available on all commands, ideally parsed and set up before the command is run. So then one could alias myproject-spack 'spack --config ~/myproject/dotspack'
3. Search up the source tree for a dotspack directory.

I would lean toward (2) as probably the best way to do this. As long as the Bash alias works, users can easily set up as many per-project Spack-like commands as they need.

@tgamblin Do you have any preferences of how the per-project scope would be specified --- or maybe something different from 1,2,3 above?