fix(build): explicit non-private package not respected in packaging (a…

…ws#15435)

The cdk-package build tool skips packaging in private packages by
looking at the value of the "private" entry in package.json.
Currently, this entry must be missing for cdk-package to treat the
package as public, but we should also allow an explicit `false` value.

fixes aws#15203

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

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

@@ -42,7 +42,7 @@ export function isJsii(): boolean {
  * Whether this is a private package
  */
 export function isPrivate(): boolean {
-  return currentPackageJson().private !== undefined;
+  return currentPackageJson().private;
 }
 
 export interface File {