Relax conflict checks for externals (#6907)
Relax conflict checks for externals (#6907)

This updates the conflict-checking logic to require that the conflict
spec matches exactly and that all fields mentioned in the conflict
spec are present in the concretized spec in order to report a
conflict. This will automatically skip all conflict checks for
dependencies of externals (since externals strip dependencies). This
will not affect non-external packages since all fields and
dependencies are fully specified for such packages.
krafczyk/spack,krafczyk/spack,mfherbst/spack,EmreAtes/spack,LLNL/spack,LLNL/spack,krafczyk/spack,iulian787/spack,matthiasdiener/spack,iulian787/spack,EmreAtes/spack,tmerrick1/spack,iulian787/spack,matthiasdiener/spack,mfherbst/spack,EmreAtes/spack,LLNL/spack,iulian787/spack,mfherbst/spack,mfherbst/spack,matthiasdiener/spack,tmerrick1/spack,matthiasdiener/spack,tmerrick1/spack,LLNL/spack,tmerrick1/spack,matthiasdiener/spack,krafczyk/spack,mfherbst/spack,tmerrick1/spack,krafczyk/spack,LLNL/spack,EmreAtes/spack,iulian787/spack,EmreAtes/spack