add pacman to known providers

Yeah that was the problem I was facing. The idea behind `available_versions` seems to get a list of all the different installable versions of one package, but of course `pacman -Ss` returns all potential packages matching the given string, but only one version of each package.

So I decided to not turn them into version_numbers, because that would be meaningless and just putting the search result might into available_version might be helpful in the case of ambiguity. 

The only place where `available_versions` is used is in `package_available` a few lines above