Improve diagnostics for duplicated names

diag_test/test_template.rs

@@ -0,0 +1,35 @@
+use rhai::plugin::*;
+
+#[derive(Clone)]
+pub struct Point {
+    x: f32,
+    y: f32,
+}
+
+#[export_module]
+pub mod test_module {
+    #[rhai_mod(name = "bar")]
+    pub mod test_mod {
+        #[rhai_fn(name = "foo")]
+        pub fn test_fn(input: Point) -> bool {
+            input.x > input.y
+        }
+
+        #[rhai_fn(return_raw)]
+        pub fn test_fn_raw(input: Point) -> Result<rhai::Dynamic, Box<rhai::EvalAltResult>> {
+            Ok(Dynamic::from(input.x > input.y))
+        }
+    }
+}
+
+fn main() {
+    let n = Point {
+        x: 0.0,
+        y: 10.0,
+    };
+    if test_module::test_mod::test_fn(n) {
+        println!("yes");
+    } else {
+        println!("no");
+    }
+}