module: drop support for extensionless main entry points in esm

PR-URL: https://github.com/nodejs/node/pull/31415


Reviewed-By: Guy Bedford <guybedford@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Bradley Farias <bradley.meck@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>