edition 2018 support for mod #[path=] attribute

[burrbull]

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.

[crlf0710]

Just run into this issue with winit 2018 migration. cc #61924 rust-windowing/winit#912

[crlf0710]

@petrochenkov do you think this is a C-bug or C-enhancement?

[petrochenkov]

do you think this is a C-bug or C-enhancement?

¯_(ツ)_/¯
I always forget how exactly #[path] works, I only remember that it doesn't work consistently, but changing that is a breaking change.

[crlf0710]

Is it a breaking change adding an extra candidate?

[petrochenkov]

cc @cramertj (perhaps you remember the details?)

[cramertj]

Yeah, it's a totally confusing mess. The reference's section on this should be correct, though (it isn't, it's missing some things).

[ehuss]

The reference is incomplete. See rust-lang/reference#573, which I believe is exactly this issue.