Consistently use quote! when emitting 'source'

When a macro generates part of the derive input, the call-site hygiene
may be different than the hygiene of a field. Therefore, we need to
be sure to use the same hygiene information for any identifiers we
generate, instead of relying on the hygiene from a particular span
via `quote_spanned!`

tests/test_source.rs

@@ -48,3 +48,16 @@ fn test_boxed_source() {
     let error = BoxedSource { source };
     error.source().unwrap().downcast_ref::<io::Error>().unwrap();
 }
+
+macro_rules! error_from_macro {
+    ($($variants:tt)*) => {
+        #[derive(Error)]
+        #[derive(Debug)]
+        pub enum MacroSource {
+            $($variants)*
+        }
+    }
+}
+
+// Test that we generate impls with the proper hygiene
+error_from_macro!(#[error("Something")] Variant(#[from] io::Error));