Add vf2pp_helpers subpackage to wheel

[rossbar]

Follow up to #5973, which didn't actually fix the problem with the doc build.

After more debugging, I determined that the problem was the vf2pp_helpers subpackage wasn't actually included in the wheel. I missed this in my first round of debugging that led to my mis-diagnosis in #5973 likely from accidentally using pip -e and still building the docs from the source directory. We should probably add/update at least one CI job to not use editable installs to catch this problem in the future, though it's worth noting that the vf2pp_helpers subpackage appears to be unique - i.e. there are no other "helper" subpackages that are not included in an init.py somewhere.

In the slightly longer term, it might be better to refactor the vf2pp_helpers subpackage into a submodule instead (e.g. _vf2pp_helpers.py) which should remove the need to explicitly list the extra directory in the setup.py. That should be straightforward, but will touch a lot of import lines, so I will leave it to a different PR.

[rossbar]

We should probably add/update at least one CI job to not use editable installs to catch this problem in the future

A quick follow-up here: I just checked our CI and we do already do this, so the fact that nothing was failing could be due to the test suite being run in the source directory in CI runs. I'm not really sure.

[dschult]

I we make a new PR to move these helper files into a single module, we should consider putting them all into the vf2pp.py module. I don't know how long that will make that file, but we should at least consider it.

[dschult]

I think I will go ahead and merge this since other packages are waiting on it... and the merge is what gives the definitive test :}