Lint on `mod foo {}` inside of `foo.rs`

[estebank]

One newcomer mistake is getting confused by the module system ending up with crate::module::module by having pub mod module {} inside of module.rs. We should have a lint that detects this case and gently nudges the user towards the more likely intended structure.

[hanna-kruppe]

Clippy has https://rust-lang.github.io/rust-clippy/master/index.html#module_inception — could be uplifted.

[chenyukang]

it's reported here: https://github.com/rust-lang/rust-clippy/blob/428208eb9b61f44e5bbab70feea42a8d55994230/clippy_lints/src/item_name_repetitions.rs#L482-L487

what's the proper way to make this check live rustc? we add a module_inception in rustc_lint?

[estebank]

@chenyukang yeah, uplifting this to rustc_list would make sense, IMO.