Fix package_name and final_dmg inconsistency if safe name != name

If the project name has characters such as "-" in, package_name wasn't
resolving to what was created in the final output. This fix ties the two functions
together to avoid that, and also work with other packagers than "pkg".
We should just nuke `Omnibus::Compressor::DMG#final_dmg` and use `Omnibus::Compressor::DMG#package_name` throughout this class. That would be consistant with what's done in `Omnibus::Compressor::TGZ`.