Fix getPackageForModule implementation against a package root path

Summary:
Fixes #965.

**Cause**: The implementation of `context.getPackageForModule(modulePath: string)` attempts to locate a `package.json` file starting with `parsedPath.dir` — however this clobbers the path basename when `modulePath` is initially the package directory.

e.g. for the path `/root/node_modules/ipld/dag-cbor` (a package root):

- `/root/node_modules/ipld/dag-cbor/package.json` **exists**.
- `getPackageForModule('/root/node_modules/ipld/dag-cbor')` begins traversal, **incorrectly**, from `/root/node_modules/ipld`.

https://pxl.cl/2Cb1T

**Why this didn't matter until now**

- This was previously inconsequential for `mainFields` (`"browser"` field spec), since redirections ***only*** support subpaths. (`"browser"` does not describe/permit a `"."` subpath, but instead a redirection of a *file path*, e.g. `"./index.js"`.)
https://github.com/defunctzombie/package-browser-field-spec#replace-specific-files---advanced

- On fall through from `mainFields` subpath resolution (implemented via `context.redirectModulePath` and not sharing the regular `nodeModulesPath` iteration loop), `resolveModulePath` will attempt `./package.json` to call [`getPackageEntryPoint`](https://github.com/facebook/metro/blob/d0b8ea57749d081f66c62ad4a4863caff62aa1d4/packages/metro-resolver/src/PackageResolve.js#L23).

Note: In tests, this behaviour was already (necessarily) added in D43055160, but did not line up!:
https://github.com/facebook/metro/blob/d0b8ea57749d081f66c62ad4a4863caff62aa1d4/packages/metro-resolver/src/__tests__/utils.js#L102

Changelog: **[Experimental]** Fix `package.json` discovery against root package specifiers for Package Exports

(⬆️ Would previously fall-through to file-based resolution.)

Reviewed By: robhogan

Differential Revision: D44877146

fbshipit-source-id: bb02d81b9e7d2ef71846d492cfbc38c85a4bf98b

packages/metro-resolver/src/__tests__/utils.js

@@ -99,7 +99,7 @@ export function createPackageAccessors(
   const getPackageForModule = (modulePath: string) => {
     const parsedPath = path.parse(modulePath);
     const root = parsedPath.root;
-    let dir = path.join(parsedPath.dir, parsedPath.name);
+    let dir = path.join(parsedPath.dir, parsedPath.base);
 
     do {
       if (path.basename(dir) === 'node_modules') {

packages/metro/src/node-haste/DependencyGraph.js

@@ -152,7 +152,8 @@ class DependencyGraph extends EventEmitter {
   _getClosestPackage(filePath: string): ?string {
     const parsedPath = path.parse(filePath);
     const root = parsedPath.root;
-    let dir = parsedPath.dir;
+    let dir = path.join(parsedPath.dir, parsedPath.base);
+
     do {
       // If we've hit a node_modules directory, the closest package was not
       // found (`filePath` was likely nonexistent).