### What it does
Checks that module layout uses only `mod.rs` files.

### Why is this bad?
Having multiple module layout styles in a project can be confusing.

### Example
```
src/
  stuff/
    stuff_files.rs
  stuff.rs
  lib.rs
```
Use instead:
```
src/
  stuff/
    stuff_files.rs
    mod.rs
  lib.rs
```