[Bug] Infinite loop with mkdirp on n_m

[arcanis]

Describe the bug

When operating from within a PnPify environment the node_modules folder exists, but creating a folder inside it (in particular node_modules/.cache) isn't possible without triggering a ENOENT.

In practice it translates into an infinite loop with the mkdirp package, triggered in particular by Webpack operating in production mode (because Terser uses the cache).

The fix would be to silently create the node_modules folder when a dot-file (or dot-folder) is created within node_modules.

To Reproduce

await packageJsonAndInstall({
  dependencies: {
    [`@yarnpkg/pnpify`]: `latest`,
    [`mkdirp`]: `0.5.1`,
  },
});

const res = await node(`
  (async () => {
    require('@yarnpkg/pnpify').patchFs();

    const mkdir = new Promise((resolve, reject) => {
      require('mkdirp')('node_modules/.cache', err => {
        err ? resolve('Error:  ' + err.message) : resolve(true);
      });
    });

    const timeout = new Promise(resolve => {
      setTimeout(() => resolve('Timeout!'), 1000);
    });

    return await Promise.race([mkdir, timeout]);
  })()
`);

expect(res).toEqual(true);

[yarnbot]

We couldn't reproduce your issue (all the assertions passed on master).