Support multiple esmf versions in unified environment / updates of neptune-dev, unified-dev, skylab-dev templates (add `%oneapi`)

Summary

This PR makes it possible to use different ESMF versions for virtual packages within one large environment. NEPTUNE needs esmf@8.7.0b04 (for now), while the other applications need esmf@8.6.1.

There are also cosmetic (indentation) changes for the skylab-dev and unified-dev templates, and an update of the neptune-dev template. I've turned off the Python variant for neptune-env when built as part of the unified environment due to numerous version conflicts and duplicates. We'll want to look at those later, after #1138 was merged and after the py-xnrl package dependencies are updated (ongoing at NRL).

Testing

Concretize both the neptune-dev (NEPTUNE standalone) template and the unified-dev template on my laptop
Ran CI tests (only builds unified-dev at the moment)

Applications affected

None (no changes from the current status quo)

Systems affected

None

Dependencies

None

Issue(s) addressed

Resolves https://github.com/JCSDA/spack-stack/issues/1154

Checklist

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

JCSDA / spack-stack