chore(ubergen): make rosetta fixtures submodule aware (aws#17438)

Currently ubergen bundles all fixtures into the same `rosetta` folder.
This means that all fixtures in files called `default.ts-fixture` replace
each other as they are found and vie to be the last `default.ts-fixture`
standing in the `rosetta` folder. This causes unintended compiliation
failures for `yarn rosetta:extract --compile` in projects that rely on
ubergen, since they will be copied into the wrong `default.ts-fixture`.

The solution here in jsii is to find fixtures based on submodules: aws/jsii#3131.
This PR updates ubergen to construct the necessary subfolders in
`rosetta` so that the expected submodule fixtures can be found. 

----

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

tools/@aws-cdk/ubergen/bin/ubergen.ts

@@ -254,8 +254,9 @@ async function combineRosettaFixtures(libraries: readonly LibraryReference[]) {
 
   for (const library of libraries) {
     const packageRosettaDir = path.join(library.root, 'rosetta');
+    const uberRosettaTargetDir = library.shortName === 'core' ? uberRosettaDir : path.join(uberRosettaDir, library.shortName.replace(/-/g, '_'));
     if (await fs.pathExists(packageRosettaDir)) {
-      await fs.copy(packageRosettaDir, uberRosettaDir, {
+      await fs.copy(packageRosettaDir, uberRosettaTargetDir, {
         overwrite: true,
         recursive: true,
       });