parser: Fix resolution of #[path] dependencies from certain modules.

We need the current mod dir to resolve #[path], not the one we get for
rust 2018.

Fixes #599

tests/rust/mod_2018/src/nested.rs

@@ -1 +1,4 @@
 pub mod other;
+
+#[path = "other2.rs"]
+pub mod other2;

tests/rust/mod_2018/src/other2.rs

@@ -0,0 +1,7 @@
+#[repr(C)]
+pub struct ExportMe2 {
+    val: u64
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn export_me_2(_: *mut ExportMe2) { }