This is how I see the logic of this new feature you're adding:
When you have a folder named "mod" which contains a file named package.d, the mod folder becomes conceptually a file with same name and the extension "d". I.e. the folder conceptually becomes a module, which you can then import to other modules. All the code contained within all the files contained within the mod folder becomes conceptually the content of this imaginary mod.d file. Therefore it is only logical that all the code within this imaginary mod.d file would have access to 'package'-labeled symbols that are in files which are located under the same folder under which this imaginary mod.d file is.
