Add bash completion for pacman functions

Bash completion was not available for Aura when using pacman specific
options (e.g, aura -S foo<tab><tab>). To enable completion, the
following chages have been made:

* Copied the following functions from the existing pacman completions
  file: _arch_compgen, _arch_ptr2comp, _arch_incomp, _pacman_pkg,
  _pacman, _pacman_file
* Added the "_aura" prefix to each of the "_pacman" function names to
  preserve naming conventions (e.g., _pacman -> _aura_pacman)