On *BSD systems, we can `open(2)` a directory and directly `read(2)` from it due to an old tradition. We should avoid doing so by explicitly calling `fstat(2)` to check the type of the opened file.
Opening a directory as a module file can't always be avoided. Even when there's no "path" attribute trick involved, there can always be a *directory* named `my_module.rs`.
Incidentally, remove unnecessary mutability of `&self` from `io::fs::File::stat()`.