Omnibus aware paths

This PR modifies the path helpers to be aware of omnibus_dir being set.

Unfortunately the recipe also has to tell the packaging step where the files have been placed outside the omnibus_dir (I use this to place init scripts for Omnibus packages). I did consider having the path helpers track where they had placed files, but it was ugly and impractical.

The parent Omnibus recipe gains a new property: omnibus_additional_paths. This is an array of paths that should also be added to the package. It's better to have this on the parent recipe, so it's obvious to anyone who looks what paths get packaged without having to investigate each of the child recipes.