This would allow someone to package any directory... however, inside `Mix.Package`, you access `Mix.project`, which contains information about the current project and not the one on the path you are calling `mix package`. So we have two options: to get rid of this functionality or to not access `Mix.project` inside `Mix.Package`. I prefer the latter, so we should just assume in `Mix.Package` that the compile path is "ebin" and raise if "ebin" is not there and if there isn't an `.app` file.