module: throw an error for invalid package.json main entries

We currently ignore invalid `main` entries in package.json files.
This does not seem to be very user friendly as it's certainly an
error if the `main` entry is not a valid file name. So instead of
trying to resolve the file otherwise, throw an error immediately to
improve the user experience.
To keep it backwards compatible `index.js` files in the same directory
as the `package.json` will continue to be resolved instead but that
behavior is now deprecated.

PR-URL: https://github.com/nodejs/node/pull/26823
Fixes: https://github.com/nodejs/node/issues/26588


Reviewed-By: Guy Bedford <guybedford@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>