Description

Note: this isn't going to pass until after #2946

Dev notes & details

• We had the odd situation where the install directories (GNUInstallDirs) were pretty flexible. But it wasn't quite flexible enough for the usual Windows install layout (see https://github.com/psi4/psi4/blob/master/conda/win/meta.yaml#L32-L41). And if you got those all in the right place, the paths computation in run_psi4.py (-> bin/psi4) and psi4/__init__.py wasn't quite as flexible as GNUInstallDirs allowed. So I modernized the lot with pathlib and hopefully systemitized it a bit, too.
• There's a weird case on linux c-f where we're in a git repo, but it's not the psi git repo. Thus versioning is much confused. The versioner can now escape from that case and version as if tarball.

Checklist

• Tests added for any new features
• All or relevant fraction of full tests run
• works for lin, mac, win conda-builds

Status

• Ready for review
• Ready for merge