fix(rosetta): 'extract' does not translate samples in submodule READM…

…Es (#2744)

Now that the CDK release branch contains the ubergen [fix](aws/aws-cdk@e047bda) that aligns rosseta fixtures, we can try and re-introduce this fix. 

---

By submitting this pull request, I confirm that my contribution is made under the terms of the [Apache 2.0 license].

[Apache 2.0 license]: https://www.apache.org/licenses/LICENSE-2.0

packages/jsii-rosetta/lib/jsii/assemblies.ts

@@ -68,6 +68,18 @@ export function allSnippetSources(
     });
   }
 
+  for (const [submoduleFqn, submodule] of Object.entries(
+    assembly.submodules ?? {},
+  )) {
+    if (submodule.readme) {
+      ret.push({
+        type: 'markdown',
+        markdown: submodule.readme.markdown,
+        where: removeSlashes(`${submoduleFqn}-README`),
+      });
+    }
+  }
+
   if (assembly.types) {
     Object.values(assembly.types).forEach((type) => {
       emitDocs(type.docs, `${assembly.name}.${type.name}`);

packages/jsii-rosetta/test/jsii/assemblies.test.ts

@@ -29,6 +29,33 @@ test('Extract snippet from README', () => {
   expect(snippets[0].visibleSource).toEqual('someExample();');
 });
 
+test('Extract snippet from submodule READMEs', () => {
+  const snippets = Array.from(
+    allTypeScriptSnippets([
+      {
+        assembly: fakeAssembly({
+          submodules: {
+            'my.submodule': {
+              readme: {
+                markdown: [
+                  'Before the example.',
+                  '```ts',
+                  'someExample();',
+                  '```',
+                  'After the example.',
+                ].join('\n'),
+              },
+            },
+          },
+        }),
+        directory: path.join(__dirname, 'fixtures'),
+      },
+    ]),
+  );
+
+  expect(snippets[0].visibleSource).toEqual('someExample();');
+});
+
 test('Extract snippet from type docstring', () => {
   const snippets = Array.from(
     allTypeScriptSnippets([