Improve out of line module resolution

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``.

1 files changed, 14 insertions, 0 deletions

diff --git a/src/test/mod_resolver.rs b/src/test/mod_resolver.rs
index ec9ed0f0b8d..fcff6d14e6f 100644
--- a/src/test/mod_resolver.rs
+++ b/src/test/mod_resolver.rs
@@ -50,3 +50,17 @@ fn skip_out_of_line_nested_inline_within_out_of_line() {
         &["tests/mod-resolver/skip-files-issue-5065/one.rs"],
     );
 }
+
+#[test]
+fn fmt_out_of_line_test_modules() {
+    // See also https://github.com/rust-lang/rustfmt/issues/5119
+    verify_mod_resolution(
+        "tests/mod-resolver/test-submodule-issue-5119/tests/test1.rs",
+        &[
+            "tests/mod-resolver/test-submodule-issue-5119/tests/test1.rs",
+            "tests/mod-resolver/test-submodule-issue-5119/tests/test1/sub1.rs",
+            "tests/mod-resolver/test-submodule-issue-5119/tests/test1/sub2.rs",
+            "tests/mod-resolver/test-submodule-issue-5119/tests/test1/sub3/sub4.rs",
+        ],
+    )
+}