Add a test for #3427

Author: topecongiro

tests/source/path_clarity/foo.rs

@@ -0,0 +1,2 @@
+// rustfmt-edition: 2018
+mod bar;

tests/source/path_clarity/foo/bar.rs

@@ -0,0 +1,3 @@
+pub fn fn_in_bar( ) {
+    println!( "foo/bar.rs" );
+}

tests/target/path_clarity/foo.rs

@@ -0,0 +1,2 @@
+// rustfmt-edition: 2018
+mod bar;

tests/target/path_clarity/foo/bar.rs

@@ -0,0 +1,3 @@
+pub fn fn_in_bar() {
+    println!("foo/bar.rs");
+}