Fix ordering of nested modules in non-mod.rs mods

Flatten relative offset into directory path before adding inline
(mod x { ... }) module names to the current directory path.

Fix #55094

src/test/ui/non_modrs_mods_and_inline_mods/non_modrs_mods_and_inline_mods.rs

@@ -0,0 +1,5 @@
+// compile-pass
+
+mod x;
+
+fn main() {}

src/test/ui/non_modrs_mods_and_inline_mods/x.rs

@@ -0,0 +1,5 @@
+// ignore-test: not a test
+
+pub mod y {
+    pub mod z;
+}

src/test/ui/non_modrs_mods_and_inline_mods/x/y/z/mod.rs

@@ -0,0 +1 @@
+// ignore-test: not a test