Move top-level files to mod.rs when applicable (#13732)

There are two modules in `clippy_utils` that are currently in the form
of:

```
src/
  | ast_utils/
  | ty/
  |
  | ast_utils.rs
  | ty.rs
```

This PR moves the top-level modules to become `mod.rs`, within their
respective folders.

```
src/
  | ast_utils/
  |   | mod.rs
  |
  | ty/
  |   | mod.rs
```

This reduces clutter in the `src` folder, and makes it easier to find
related files in certain editors.[^0] This also appears to be the
standard used in other crates in this repository, though I looked very
briefly.

I do realize that this is a style / opinionated change, so I'll close it
if it receives much push-back. :)

[^0]: I use VSCode, which groups all folders together and all files
separately. This means that `ty.rs` is quite "far" away from the `ty/`
folder, which makes it move difficult to navigate between the two.

```
changelog: none
```

- \[x] `cargo test` passes locally
- \[x] Run `cargo dev fmt`