add utility for listing pkg versions for spack-stack wiki

[AlexanderRichert-NOAA]

Summary

This PR adds a utility for getting a list of package versions which can be copied/pasted into the wiki: https://github.com/JCSDA/spack-stack/wiki/Package-versions

Testing

Used to populate https://github.com/JCSDA/spack-stack/wiki/Package-versions for 1.6.0 through 1.8.0.

Applications affected

n/a

Systems affected

n/a

Dependencies

none

Issue(s) addressed

Fixes #1320

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.

[climbfuji]

Do you want to add this to the util tests in CI (just verifying that it runs successfully, not validating the output)?

[AlexanderRichert-NOAA]

Not really, if that's okay with you. It's something that will get run manually once per release, so I'd rather put the development time into fixing it if/when it breaks.

[RatkoVasic-NOAA]

Looks straightforward, approved.

[climbfuji]

Not really, if that's okay with you. It's something that will get run manually once per release, so I'd rather put the development time into fixing it if/when it breaks.

Fine with me!