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.