bash completion: fixed invalid identifier (#4079)
bash completion: fixed invalid identifier (#4079)

* bash completion: fixed `_spack_create-db-tarball': not a valid identifier

* bash completion: dashes are translated to underscores

This also fixes the name of the subfunction to be called, as apparently
it was not updated after moving the command `create-db-tarball`.

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