child_process: use module.exports.spawn in fork and execFile

[robertsLando]

fork() and execFile() call the locally-scoped spawn() function
directly, bypassing any wrapping applied to child_process.spawn via
module exports.

This is inconsistent with exec(), which correctly uses
module.exports.execFile() (see lib/child_process.js:236).

The problem

APM tools (New Relic, Datadog), security monitors, and testing
frameworks commonly wrap child_process.spawn on exports to intercept
child process creation. With the current code, processes spawned
through fork() and execFile() silently bypass these wrappers, while
exec() correctly goes through them.

const cp = require('child_process');
const originalSpawn = cp.spawn;

cp.spawn = function (...args) {
  console.log('spawn intercepted');
  return originalSpawn.apply(this, args);
};

cp.exec('echo hi');       // intercepted (exec → module.exports.execFile → ... → spawn)
cp.fork('./worker.js');   // NOT intercepted (fork → local spawn)
cp.execFile('ls', []);    // NOT intercepted (execFile → local spawn)

The fix

Change fork() and execFile() to call module.exports.spawn()
instead of the locally-scoped spawn(), consistent with the pattern
already used by exec().

Refs: yao-pkg/pkg#231

[Renegade334]

For context:

This is inconsistent with exec(), which correctly uses
module.exports.execFile()

This was a leftover from when the child_process methods existed as module.exports properties rather than as module-declared functions (aa00968). AFAIA, the decision to leave this here was a convenience thing to avoid branch conflicts, not a statement of intent for facilitating the monkeypatching of API internals.