Fix #430: edit -f fails with UnknownPackageError
Fix #430: edit -f fails with UnknownPackageError

- Recent external repo refactoring made `repo_for_pkg` raise an error
  when the package was not known.

- Correct behavior is to return the highest precedence repo.

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