Fix mismatched metro-transform-worker version - use absolute worker…

… paths

Summary:
Issues such as #1017 (and similar previously) have had users running into broken setups due to "multiple versions of Metro" installed.

Ideally, multiple Metros wouldn't be a problem. Metro packages already specify explicit, pinned dependencies on each other, and well-behaved package resolutions should stay within those disconnected graphs of Metro versions.

In particular `metro` already specifies its dependency on `metro-transform-worker`, so it wasn't obvious how we were "escaping" from the correct `metro` and ending up inside the wrong `metro-transform-worker` here. I was able to reproduce #1017 with Berry and debug it.

It turns out that the problem is the use of the (undocumented) `transformer.workerPath` option, which defaults to `'metro/src/DeltaBundler/Worker'`. We pass that to `jest-worker`, which forks a child process `jest-worker/build/workers/processChild`, which resolves the given path *relative to `jest-worker`* - so usually resolves the *hoisted* `metro`, whatever that happens to be.

This fixes the issue by resolving the path from the parent process, within `metro`, and passing an absolute path to `jest-worker`.

Changelog:
```
 - **[Fix]**: Incorrect worker resolution when multiple `metro` versions are installed.
```

Reviewed By: huntie

Differential Revision: D47209874

fbshipit-source-id: 5aa101852e9d33af6f41c118252d9874701a01be

packages/metro/src/DeltaBundler/WorkerFarm.js

@@ -39,10 +39,11 @@ class WorkerFarm {
   constructor(config: ConfigT, transformerConfig: TransformerConfig) {
     this._config = config;
     this._transformerConfig = transformerConfig;
+    const absoluteWorkerPath = require.resolve(config.transformer.workerPath);
 
     if (this._config.maxWorkers > 1) {
       const worker = this._makeFarm(
-        this._config.transformer.workerPath,
+        absoluteWorkerPath,
         ['transform'],
         this._config.maxWorkers,
       );
@@ -107,7 +108,7 @@ class WorkerFarm {
   }
 
   _makeFarm(
-    workerPath: string,
+    absoluteWorkerPath: string,
     exposedMethods: $ReadOnlyArray<string>,
     numWorkers: number,
   ): any {
@@ -117,7 +118,7 @@ class WorkerFarm {
       FORCE_COLOR: 1,
     };
 
-    return new JestWorker(workerPath, {
+    return new JestWorker(absoluteWorkerPath, {
       computeWorkerKey: this._config.stickyWorkers
         ? // $FlowFixMe[method-unbinding] added when improving typing for this parameters
           // $FlowFixMe[incompatible-call]