chore: copy fewer files in MonoCDK (aws#8626)

Stop copying "output" files (`.d.ts` and `.js` files corresponding to a
`.ts` file) when preparing `monocdk-experiment` for build. This saves
some copy operations and avoids copying artifacts that include non
rewritten import statements.


----

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

packages/monocdk-experiment/build-tools/gen.ts

@@ -273,6 +273,13 @@ async function copyOrTransformFiles(from: string, to: string, libraries: readonl
   const promises = (await fs.readdir(from)).map(async name => {
     if (shouldIgnoreFile(name)) { return; }
 
+    if (name.endsWith(".d.ts") || name.endsWith(".js")) {
+      if (await fs.pathExists(path.join(from, name.replace(/\.(d\.ts|js)$/, ".ts")))) {
+        // We won't copy .d.ts and .js files with a corresponding .ts file
+        return;
+      }
+    }
+
     const source = path.join(from, name);
     const destination = path.join(to, name);