mpich: Add `external find` support (#18330)
mpich: Add `external find` support (#18330)

* Adding external support to mpich

* Removing debugging print statement.
LLNL/spack,LLNL/spack,iulian787/spack,iulian787/spack,iulian787/spack,LLNL/spack,LLNL/spack,LLNL/spack,iulian787/spack,iulian787/spack