chore(cdk-build-tools): allow packing of private packages for local testing (#32644)

### Reason for this change

Sometimes there is a need to pack a private package so it can be tested as a package locally.
For example when a package is not released during development.
Previously it was not possible to pack a private package with our tooling.

### Description of changes

Adds a `--private` flag to the `cdk-package` command to allow force packing of private packages.

### Describe any new or updated permissions being added

n/a

### Description of how you validated changes

Manually used the new flag to pack a private package.

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

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

Author: mrgrain

tools/@aws-cdk/cdk-build-tools/bin/cdk-package.ts

@@ -24,6 +24,7 @@ async function main() {
     })
     .option('pre-only', { type: 'boolean', default: false, desc: 'run pre package steps only' })
     .option('post-only', { type: 'boolean', default: false, desc: 'run post package steps only' })
+    .option('private', { type: 'boolean', default: false, desc: 'Also package private packages for local usage' })
     .argv;
 
   if (args['pre-only'] && args['post-only']) {
@@ -43,8 +44,9 @@ async function main() {
   const outdir = 'dist';
 
   // if this is a private module, don't package
-  if (isPrivate()) {
-    process.stdout.write('No packaging for private modules.\n');
+  const packPrivate = args.private || options.private;
+  if (isPrivate() && !packPrivate) {
+    process.stdout.write('No packaging for private modules.\nUse --private to force packing private packages for local testing.\n');
     return;
   }
 

tools/@aws-cdk/cdk-build-tools/lib/package-info.ts

@@ -205,6 +205,12 @@ export interface CDKPackageOptions {
    * Should this package be bundled. (and if so, how)
    */
   bundle?: Omit<BundleProps, 'packageDir'>;
+
+  /**
+   * Also package private packages for local usage.
+   * @default false
+   */
+  private?: boolean;
 }
 
 /**