packer -Syu package_name does not install package_name

[notbenh]

Currently with pacman you can both update and then install a package in a single step via -Syu package_name, though this convention does not seem to be implemented in packer.

[gavinhungry]

@notbenh, I fixed this in my fork - https://github.com/gavinhungry/packer/commit/f9a533965a4adcaf673f0079920b1d709f7fd158 - I'd submit a pull request here but I'm not sure how far the context has diverged.

Should work with -Sy, -Syu, -Syy and -Syyu.