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.
