fs: use fs.access in fs.exists

Uses fs.access to implement fs.exists functionality. Fixes a issue,
when a file exists but user does not have privileges to do stat on the
file.

Fixes: https://github.com/nodejs/node/issues/17921

Backport-PR-URL: https://github.com/nodejs/node/pull/19654
PR-URL: https://github.com/nodejs/node/pull/18618


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>