Placeholder apps perform several codegens in the same output directory

[andy31415]

Reproduction steps

Code in zap_rengen_all says:

            # The name zap-generated is to make includes clear by using
            # a name like <zap-generated/foo.h>
            output_dir = os.path.join(
                'zzz_generated', 'placeholder', example_name, 'zap-generated')
            template = 'examples/placeholder/templates/templates.json'

            targets.append(ZAPGenerateTarget(filepath, output_dir=output_dir))
            targets.append(
                ZAPGenerateTarget(filepath, output_dir=output_dir, template=template))
            continue

This makes the same output directory be used for 2 separate generations. Ideally we should only have a single output directory.