Add --all argument to `spack dependents`
Add --all argument to `spack dependents`

--all causes spack dependents to list all possible dependents for a
package, rather than actual dependents for an installed spec.

lgarren/spack,EmreAtes/spack,mfherbst/spack,tmerrick1/spack,mfherbst/spack,matthiasdiener/spack,EmreAtes/spack,skosukhin/spack,skosukhin/spack,matthiasdiener/spack,mfherbst/spack,mfherbst/spack,TheTimmy/spack,krafczyk/spack,iulian787/spack,LLNL/spack,tmerrick1/spack,TheTimmy/spack,lgarren/spack,matthiasdiener/spack,iulian787/spack,EmreAtes/spack,TheTimmy/spack,lgarren/spack,mfherbst/spack,lgarren/spack,TheTimmy/spack,iulian787/spack,tmerrick1/spack,matthiasdiener/spack,tmerrick1/spack,EmreAtes/spack,LLNL/spack,LLNL/spack,krafczyk/spack,skosukhin/spack,EmreAtes/spack,iulian787/spack,LLNL/spack,skosukhin/spack,tmerrick1/spack,iulian787/spack,lgarren/spack,matthiasdiener/spack,skosukhin/spack,TheTimmy/spack,LLNL/spack,krafczyk/spack,krafczyk/spack,krafczyk/spack