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

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.

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 {