edition 2018 support for mod #[path=] attribute

If module files have edition 2018 style file structure and try to import one of them by path attribute:
```
#[path=modname.rs] 
mod modname;
```
compiler tries to find submodules in submodname.rs or submodname/mod.rs, but not in
**modname**/submodname.rs where it is really located.