When chaining against chained envs, get upstreams+envrepo's

[AlexanderRichert-NOAA]

Summary

This PR makes the env chaining logic in spack stack create env --upstream more robust, specifically by recursively checking to see whether the upstream environments are themselves chained environments and adding those to the new environment's list of upstreams. It also automatically populates an envrepo/ repo directory from those found in the upstream environments.

I've tried to support a couple users now in chaining against, e.g., upp-addon-env, and it quickly gets complicated/manual labor-intensive, so these changes should make it much easier for developers to deploy their own test environments.

Testing

Tested on personal machine & Gaea C5. Includes unit testing.

Applications affected

GSI, UWM, UPP, and anything else that uses addon envs.

Systems affected

all

Dependencies

none

Issue(s) addressed

none

Checklist

• This PR addresses one issue/problem/enhancement, or has a very good reason for not doing so.
• These changes have been tested on the affected systems and applications.
• All dependency PRs/issues have been resolved and this PR can be merged.

[srherbener]

This looks good to me, but I'm not expert in the chained environment feature. Thanks for adding this in!