contrib/bash-completions: use package naming conventions

This naming scheme supports auto-detection and on-demand loading of completions.

See
https://github.com/scop/bash-completion/blob/ba109693ee2284f6a82f8f0e1563baf071252df9/README.md#faq,
section "Where should I put it to be sure that interactive bash shells will find it and source it".

Previously, distro package maintainers had to rename these files manually.

Author: erikarvstedt