module: throw on require('./path.mjs');

This is an extremely important part of the ESM implementation
that should have been unflagged as a breaking change in v12.0.0
to allow us to unflag ESM in Node.js 12.x before LTS. Assuming we
can get consensus on this behavior I would argue that this Semver-Major
behavior change could be viewed as a Semver-Patch fix in v12.0.1

PR-URL: https://github.com/nodejs/node/pull/27417


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Jan Krems <jan.krems@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ujjwal Sharma <usharma1998@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>