Add directory tree to mod exercise solution (#1602)

The solution to the modules exercise comes with a bunch of files. I
think it would help the students to get a quick overview of the
directory structure we've chosen first.

src/modules/solution.md

@@ -16,6 +16,16 @@
 // limitations under the License.
 -->
 
+```ignore
+src
+├── main.rs
+├── widgets
+│   ├── button.rs
+│   ├── label.rs
+│   └── window.rs
+└── widgets.rs
+```
+
 ```rust,ignore
 // ---- src/widgets.rs ----
 mod button;