MPI_VARIANTS.txt

From conda-forge meeting discussion, the proposal was change to: basically the same as now, but use build strings directly instead of setting any features:

nothing to do with features in any package
mpi metapackage sets build string for each variant
mpi providers depend on metapackage with buildstring, e.g. - mpi 1.0 mpich, rather than relying on features for mutual exclusivity
downstream packages include mpi variant in build string manually, e.g.
{% set mpi = os.environ['MPI_VARIANT'] %}
build:
  number: {{ build }}
    string: {{ mpi }}_{{ build }}
    downstream packages depend directly on mpi provider, not mpi metapackage
    users just install packages, but can pin metapackage if they so choose
    BTW, this proposal means that downstream packages can begin adopting mpi variants right away, since there aren't features to wait for in the upstream packages. Only the mutual-exclusivity provided by the metapackage is something to wait for, but that's independent of changes to the packages.