Redefine variants in neptune-env virtual package

[climbfuji]

Summary

This PR defines three variants in the neptune-env virtual package: espc, python, xnrl. In the unified environment and in the skylab environment, the variants espc and xnrl are off, the python variant is on. This is so that py-fortranformat is available in those environments (required by pyiri-jedi). In the neptune standalone environment, all three variants are on. Note that the xnrl variant requires the python variant.

We can debate if we should move some of the Python packages from the python variant to the xnrl variant, but since these are in the unified environment anyway (because some other package needs them), it doesn't really matter.

Testing

I concretize the neptune standalone environment and the unified environment on my laptop and verified that there are no duplicate packages or conflicts. I also tested that specifying +xnrl ~python throws an error. Further testing will be done in CI (see below).

Applications affected

There are no changes to the neptune standalone environment, only to the unified environment and the skylab environment (if that is still used).

Systems affected

n/a

Dependencies

n/a

Issue(s) addressed

Need py-fortranformat in unified environment.

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.

[eap]

Thanks for this. Good idea to define this as a variant rather than splitting the package.