PMIx: add PMIx package to Spack (#5426)
PMIx: add PMIx package to Spack (#5426)

Open MPI has been relying on versions of PMIx since the 2.0.0
release.  Although the embedded version can be used, it makes
more sense long term for Spack to know about the PMIx package,
and to build Open MPI against PMIx as an external package.

There are other software components that may eventually be
reliant on PMIx including MPICH, GASNet, the OpenSHMEM reference
implementation, etc.

This is also partly motivated by the desire to have ECP ST
projects to be released through Spack.

Signed-off-by: Howard Pritchard <e96a0dc0e8681ea6cb2efe4a406fddb7f3896a73@lanl.gov>
lgarren/spack,skosukhin/spack,mfherbst/spack,lgarren/spack,skosukhin/spack,tmerrick1/spack,lgarren/spack,skosukhin/spack,matthiasdiener/spack,iulian787/spack,EmreAtes/spack,tmerrick1/spack,matthiasdiener/spack,LLNL/spack,LLNL/spack,skosukhin/spack,lgarren/spack,EmreAtes/spack,krafczyk/spack,iulian787/spack,matthiasdiener/spack,LLNL/spack,krafczyk/spack,TheTimmy/spack,EmreAtes/spack,LLNL/spack,iulian787/spack,skosukhin/spack,mfherbst/spack,EmreAtes/spack,iulian787/spack,tmerrick1/spack,TheTimmy/spack,lgarren/spack,tmerrick1/spack,TheTimmy/spack,LLNL/spack,krafczyk/spack,matthiasdiener/spack,mfherbst/spack,krafczyk/spack,tmerrick1/spack,iulian787/spack,TheTimmy/spack,EmreAtes/spack,krafczyk/spack,matthiasdiener/spack,mfherbst/spack,TheTimmy/spack,mfherbst/spack