zsh_completion: Fix package completion

Make use of _devtools_completions_all_packages() instead of using
_devtools_completions_installed_packages(). The latter was removed
during an earlier review of the patch adding ZSH completion (now
6350ec04 in master) but we forgot to change two references.

Signed-off-by: Lukas Fleischer <archlinux@cryptocrack.de>
Signed-off-by: Pierre Schmitz <pierre@archlinux.de>