module: speed up package.json parsing more

Move the logic from the previous commit to C++ land in order to avoid
creating a new string when we know we won't parse it anyway.

PR-URL: https://github.com/nodejs/node/pull/15767


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>