Small pacman fixes

Thanks for your work on the pacman related predicates.

I tried it today and I noticed 2 small problems. I had to include `08-pacman.pl` in the marelle.pl file for it to work. 
And I changed to pacman command to check if a package is installed with pacman. `pacman -Qs foo` does a regex search for "foo" on package names and the description of the package. I changed it to use `pacman -Qi foo` which searches for locally installed package names ([Querying package databases with pacman](https://wiki.archlinux.org/index.php/pacman#Querying_package_databases))