Use QPL-managed QPLCore and PigeonClient (#1612)

Summary:
Pull Request resolved: https://github.com/facebook/flipper/pull/1612

* Use the QPL-managed QPLCore and PigeonClient.

**Facebook:**

js-shared/qpl and js-shared/pigeon-client are maintained by QPL in xplat/js-shared. I've updated the Metro config and tsconfig to allow the packages to be imported without breaking the open-source version.

Reviewed By: passy

Differential Revision: D24358369

fbshipit-source-id: 7f13d6c99d8ca6ebcd0e66bf0e5f25069370cce7

desktop/scripts/paths.ts

@@ -8,6 +8,7 @@
  */
 
 import path from 'path';
+import isFB from './isFB';
 
 export const rootDir = path.resolve(__dirname, '..');
 export const appDir = path.join(rootDir, 'app');
@@ -22,3 +23,4 @@ export const babelTransformationsDir = path.resolve(
   'babel-transformer',
   'src',
 );
+export const jsSharedDir = isFB ? path.resolve(rootDir, '../../js-shared') : '';