Fixes 5119
When the directory structure was laid out as follows:
```
dir
|---mod_a
| |---sub_mod_1.rs
| |---sub_mod_2.rs
|---mod_a.rs
```
And ``mod_a.rs`` contains the following content:
```rust
mod mod_a {
mod sub_mod_1;
mod sub_mod_2;
}
```
rustfmt previously tried to find ``sub_mod_1.rs`` and ``sub_mod_2.rs``
in ``./mod_a/mod_a/``. This directory does not exist and this caused
rustfmt to fail with the error message:
Error writing files: failed to resolve mod
Now, both ``sub_mod_1.rs`` and ``sub_mod_2.rs`` are resolved correctly
and found at ``mod_a/sub_mod_1.rs`` and ``mod_a/sub_mod_2.rs``.
