* openmpi: always require pmix for 4:
`~pmix` is not applicable to version 4+, since it always builds a vendored
copy of pmix (currently 3.2.3).
* pmix: relax version requirements
When the version range was specified, newer versions didn't exist.
Also use normalized spack versions rather than artificial .9.9 /.0.0.
* openmpi: restrict pmix versions
pmix option isn't available for OpenMPI@1, and according to
https://github.com/open-mpi/ompi/issues/7988 , OpenMPI 4.0.1 will not
build with pmix@3.1.5.
* pmix: add newer versions
* OpenMPI: re-express conflicts/configure logic as conditional variants
This relies partly on `self.enable_or_disable` and its ilk to emit an
empty list when the variant isn't applicable.