Unable to retreive Fortran libraries from `mpi` package providers

Summary

I ran into some hard-coded library names here:

https://github.com/spack/spack/blob/4b41b11c30bd5c40cd9bcb11c1a629df50705141/var/spack/repos/builtin/packages/mfem/package.py#L754-L759

This would fail if we used any other mpi provider not listed here. It also would remove duplication across any other project that hard-codes it. A brief search showed others.

For example, neither of these packages reference the Fortran libraries at all:

https://github.com/spack/spack/blob/develop/var/spack/repos/builtin/packages/mvapich2/package.py
https://github.com/spack/spack/blob/develop/var/spack/repos/builtin/packages/openmpi/package.py

Rationale

Lower hard-coded bleed.

Description

This should be able to retrievable from the mpi providers themselves in one of the following ways:

# Specifically get just the fortran libraries
spec["mpi"].fortran_libs

# Get all libraries C/C++/Fortran
spec["mpi"].libs

This would also need to work with spec.ld_flags as well.

Additional information

No response

General information

• I have searched the issues of this repo and believe this is not a duplicate

Start with var/spack/repos/builtin/packages/mfem/package.py at the referenced lines, then compare the mvapich2 and openmpi package definitions. Trace how MPI provider specs expose libraries and how spec.ld_flags consumes them. Done means Fortran or all MPI libraries can be retrieved from the provider without hard-coded names and the linker flags work for alternate providers.