fix(pnp): esm - return undefined source for commonjs

[merceyz]

What's the problem this PR addresses?

The ESM loader always returns a source which after nodejs/node#47999 landed causes stuff to break.

How did you fix it?

Return undefined source for commonjs modules

Checklist

• I have read the Contributing Guide.
• I have set the packages that need to be released for my changes to be effective.
• I will check that all automated PR checks pass before the PR gets reviewed.

[arcanis]

How can I repro the problem ?

[merceyz]

Use Node.js nightly and run yarn in this repo, I've added a test so we can validate it regardless of what this repo uses in the future.

[arcanis]

That's what I did, but as of 62b2cf30f2d1326dde9d4bc047f5611f17c4a20f I couldn't repro 🤔

[merceyz]

yarn was probably crashing due to other reasons when I tested it as it's fine now, it reproduces when running yarn test:integration pnp-esm.test.ts though.

I tested with nodejs/node@2557932 but can also reproduce with nodejs/node@62b2cf3.

[arcanis]

Indeed; a minimal repro is:

node --loader ./loader.mjs <(echo '!!console.log(require.cache)')

import fs from 'fs';
import {fileURLToPath} from 'url';

export async function load(urlString, context, nextLoad) {
  return {
    format: `commonjs`,
    source: fs.readFileSync(fileURLToPath(urlString), `utf8`),
    shortCircuit: true
  };
}

[arcanis]

Relevant thread: https://openjs-foundation.slack.com/archives/C053UCCP940/p1692798777127519