DIP16: Transparently substitute module with package

[Martin Nowak]

> What happen if both pkg.d and pkg/_.d exists ? If it is not in the same  
> path (think -I compiler option). In one case, this is an issue, in the  
> other this isn't.
pkg.d would always be a module, hence result in a module/package conflict.
We'd need to directly search for a subdirectory to decide whether it's a  
package.

Maybe someone with experience of big Python projects has some valuable
insights, but I think one point of adding the file inside a folder is to
prevent from unintended pickup of folders.