Lint against `mod lib;`

### What it does

Found that some beginners write `mod lib;` in `main.rs` to reference `lib.rs` contents. Definitely not what people actually want to do!

### Lint Name

_No response_

### Category

suspicious

### Advantage

_No response_

### Drawbacks

_No response_

### Example

```rust
// main.rs
mod lib;
```

Could be written as:

```rust
use this_package as lib;
```
