buffer: do deprecation warning outside `node_modules`

In addition to `--pending-deprecation`, emit a deprecation warning
for usage of the `Buffer()` constructor for call sites that are outside
of `node_modules`.

The goal of this is to better target developers, rather than
burdening users with an omnipresent and quickly ignored warning.

This implements the result of a TSC meeting discussion
from March 22, 2018.

PR-URL: https://github.com/nodejs/node/pull/19524
Refs: https://github.com/nodejs/node/issues/19079#issuecomment-375121443


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>