feat: Documentation.toJson() can handle nested submodules (#1190)

Allow users to create documentation for nested modules. Suppose a project has the following submodule structure:

```
my-project
└── foo
    └── bar
```

where `foo` and `bar` are submodules. To generate documentation for the `bar` module, call `documentation.toJson('foo.bar')`.

src/docgen/view/documentation.ts

@@ -312,22 +312,29 @@ export class Documentation {
   }
 
   /**
-   * Lookup a submodule by a submodule name.
+   * Lookup a submodule by a submodule name. To look up a nested submodule, encode it as a
+   * dot-separated path, e.g., 'top-level-module.nested-module.another-nested-one'.
    */
   private findSubmodule(assembly: reflect.Assembly, submodule: string): reflect.Submodule {
-    const submodules = assembly.submodules.filter(
-      (s) => s.name === submodule,
-    );
+    type ReflectSubmodules = typeof assembly.submodules;
+    return recurse(submodule.split('.'), assembly.submodules);
 
-    if (submodules.length === 0) {
-      throw new Error(`Submodule ${submodule} not found in assembly ${assembly.name}@${assembly.version}`);
-    }
+    function recurse(names: string[], submodules: ReflectSubmodules): reflect.Submodule {
+      const [head, ...tail] = names;
+      const found = submodules.filter(
+        (s) => s.name === head,
+      );
 
-    if (submodules.length > 1) {
-      throw new Error(`Found multiple submodules with name: ${submodule} in assembly ${assembly.name}@${assembly.version}`);
-    }
+      if (found.length === 0) {
+        throw new Error(`Submodule ${submodule} not found in assembly ${assembly.name}@${assembly.version}`);
+      }
 
-    return submodules[0];
+      if (found.length > 1) {
+        throw new Error(`Found multiple submodules with name: ${submodule} in assembly ${assembly.name}@${assembly.version}`);
+      }
+
+      return tail.length === 0 ? found[0] : recurse(tail, found[0].submodules);
+    }
   }
 
   private async createAssembly(