fix: importing aws-cdk-lib is slow (#23813)

Fixes slow import of top-level aws-cdk-lib.

At the very end of the build, the compiled `index.js` file is replaced with a version optimized for import performance.
Instead of loading submodules directly at the top level, exports are defined as getter functions.
This way they will only be required when actually imported from `aws-cdk-lib`.
Improves AWS CDK app performance by ~400ms.

Background reading: https://medium.com/trabe/exporting-getters-in-commonjs-modules-dd8f98b7d85e

----

```console
$ hyperfine -i "node -e \"const { App } = require('aws-cdk-lib');\"" "node -e \"const { App } = require('aws-cdk-lib-candidate');\"" --warmup 5
Benchmark 1: node -e "const { App } = require('aws-cdk-lib');"
  Time (mean ± σ):      1.227 s ±  0.125 s    [User: 1.102 s, System: 0.262 s]
  Range (min … max):    1.110 s …  1.508 s    10 runs
 
Benchmark 2: node -e "const { App } = require('aws-cdk-lib-candidate');"
  Time (mean ± σ):     251.9 ms ±  19.7 ms    [User: 226.6 ms, System: 51.0 ms]
  Range (min … max):   224.6 ms … 278.3 ms    10 runs
 
Summary
  'node -e "const { App } = require('aws-cdk-lib-candidate');"' ran
    4.87 ± 0.62 times faster than 'node -e "const { App } = require('aws-cdk-lib');"'
```


----

### All Submissions:

* [x] Have you followed the guidelines in our [Contributing guide?](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md)

### Adding new Construct Runtime Dependencies:

* [ ] This PR adds new construct runtime dependencies following the process described [here](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md/#adding-construct-runtime-dependencies)

### New Features

* [ ] Have you added the new feature to an [integration test](https://github.com/aws/aws-cdk/blob/main/INTEGRATION_TESTS.md)?
	* [ ] Did you use `yarn integ` to deploy the infrastructure and generate the snapshot (i.e. `yarn integ` without `--dry-run`)?

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

Author: mrgrain

packages/aws-cdk-lib/.npmignore

@@ -31,3 +31,8 @@ junit.xml
 
 # exclude source maps as they only work locally
 *.map
+
+# exclude tempory export files
+exports.d.ts
+exports.js
+exports.js.map

packages/aws-cdk-lib/package.json

@@ -38,6 +38,7 @@
     "stripDeprecated": true,
     "compressAssembly": true,
     "post": [
+      "cp exports.js index.js",
       "node ./scripts/verify-imports-resolve-same.js",
       "node ./scripts/verify-imports-shielded.js",
       "/bin/bash ./scripts/minify-sources.sh"

packages/aws-cdk-lib/scripts/minify-sources.sh

@@ -16,7 +16,7 @@
 scriptdir=$(cd $(dirname $0) && pwd)
 cd ${scriptdir}/..
 
-find . -name '*.js' ! -name '.eslintrc.js' ! -path '*node_modules*' | xargs npx esbuild \
+find . -name '*.js' ! -name 'exports.js' ! -name '.eslintrc.js' ! -path '*node_modules*' | xargs npx esbuild \
   --sourcemap \
   --platform=node \
   --format=cjs \

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

@@ -280,7 +280,14 @@ async function prepareSourceFiles(libraries: readonly LibraryReference[], packag
     './.warnings.jsii.js': './.warnings.jsii.js',
   };
 
+  // We use the index.ts to compile type definitions.
+  // At the very end we replace the compiled index.js file with our fixed version exports.js.
+  // exports.js has the top level submodules exports defined as a getter function,
+  // so they are not automatically loaded when importing from `aws-cdk-lib`.
+  // This improves AWS CDK app performance by ~400ms.
   const indexStatements = new Array<string>();
+  const exportsStatements = new Array<string>();
+
   for (const library of libraries) {
     const libDir = path.join(libRoot, library.shortName);
     const copied = await transformPackage(library, packageJson, libDir, libraries);
@@ -290,13 +297,19 @@ async function prepareSourceFiles(libraries: readonly LibraryReference[], packag
     }
     if (library.shortName === 'core') {
       indexStatements.push(`export * from './${library.shortName}';`);
+      exportsStatements.unshift(`export * from './${library.shortName}';`);
     } else {
       indexStatements.push(`export * as ${library.shortName.replace(/-/g, '_')} from './${library.shortName}';`);
+      exportsStatements.push(`Object.defineProperty(exports, '${library.shortName.replace(/-/g, '_')}', { get: function () { return require('./${library.shortName}'); } });`);
     }
     copySubmoduleExports(packageJson.exports, library, library.shortName);
   }
 
+  // make the exports.ts file pass linting
+  exportsStatements.unshift('/* eslint-disable @typescript-eslint/no-require-imports */');
+
   await fs.writeFile(path.join(libRoot, 'index.ts'), indexStatements.join('\n'), { encoding: 'utf8' });
+  await fs.writeFile(path.join(libRoot, 'exports.ts'), exportsStatements.join('\n'), { encoding: 'utf8' });
 
   console.log('\t🍺 Success!');
 }