add pacman to known providers

The above won't work as it should. `chomp(l)` is a full line such as `extra/imagemagick 6.8.9.6-1 [installed]`, but you need to return an array of `VersionNumber`s.

In addition, `pacman -Ss imagemagick` will return two entries: the one of the `imagemagick` package, and the one of `imagemagick-doc`. This in turn will trigger the warning in `available_version`, even though it shouldn't.