src: don't abort when package.json is a directory

PR-URL: https://github.com/nodejs/node/pull/18270
Fixes: https://github.com/nodejs/node/issues/8307


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Bradley Farias <bradley.meck@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Guy Bedford <guybedford@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>