Packager - Fix absolute imports on Windows

Summary:
Absolute imports on Windows were broken, I'm not 100% sure when this happens but when I tested Exponent on Windows which uses `rn-cli.config.js` with
```js
getTransformOptions() {
    return {
      reactNativePath: path.resolve('./node_modules/react-native'),
      reactPath: path.resolve('./node_modules/react'),
    };
  }
```
it seemed to use absolute paths for these modules.

I also tested absolute paths in node repl and it does work for absolute paths of different formats. `C:/root/test.js`, `/root/test.js`, `C:\root\test.js` all do resolve properly to the same module.

To fix this I resolve the absolute path using `path.resolve` on Windows. Noop on other platforms to avoid the overhead since it's not necessary.

**Test plan**
- Tested that it fixed the bug I had when running Exponent on Windows.
- Updated the absolute path test to use forward slashes since this is what happens in practice when using `getTransformOptions`. We can't test all cases on linux since adding the drive letter au
Closes https://github.com/facebook/react-native/pull/12530

Differential Revision: D4634699

Pulled By: jeanlauliac

fbshipit-source-id: 0cf6528069b79cba2e0f79f48f5a524d59b7091e

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

@@ -278,7 +278,7 @@ class ResolutionRequest {
 
   _resolveFileOrDir(fromModule: Module, toModuleName: string) {
     const potentialModulePath = isAbsolutePath(toModuleName) ?
-        toModuleName :
+        resolveWindowsPath(toModuleName) :
         path.join(path.dirname(fromModule.path), toModuleName);
 
     return this._redirectRequire(fromModule, potentialModulePath).then(
@@ -509,6 +509,16 @@ function normalizePath(modulePath) {
   return modulePath.replace(/\/$/, '');
 }
 
+// HasteFS stores paths with backslashes on Windows, this ensures the path is
+// in the proper format. Will also add drive letter if not present so `/root` will
+// resolve to `C:\root`. Noop on other platforms.
+function resolveWindowsPath(modulePath) {
+  if (path.sep !== '\\') {
+    return modulePath;
+  }
+  return path.resolve(modulePath);
+}
+
 function resolveKeyWithPromise([key, promise]) {
   return promise.then(value => [key, value]);
 }

packages/metro-bundler/src/node-haste/__tests__/DependencyGraph-test.js

@@ -2478,7 +2478,7 @@ describe('DependencyGraph', function() {
       const root = 'C:\\root';
       setMockFileSystem({
         'root': {
-          'index.js': 'require("C:\\\\root\\\\apple.js");',
+          'index.js': 'require("C:/root/apple.js");',
           'apple.js': '',
         },
       });
@@ -2493,7 +2493,7 @@ describe('DependencyGraph', function() {
             {
               id: 'C:\\root\\index.js',
               path: 'C:\\root\\index.js',
-              dependencies: ['C:\\root\\apple.js'],
+              dependencies: ['C:/root/apple.js'],
               isAsset: false,
               isJSON: false,
               isPolyfill: false,