Allow to require module path from whitelisted dependency

Required module path can be one of

- `fs` (built-in Node.js module),
- `abortcontroller-polyfill` (external main module),
- `abortcontroller-polyfill/dist/cjs-ponyfill` (external submodule).

FastBoot's build system requires only dependency whitelisting,
thus looking only for the first part of module path.

Author: bobisjan

package.json

@@ -29,8 +29,8 @@
     "chalk": "^2.0.1",
     "cookie": "^0.3.1",
     "debug": "^3.0.0",
-    "exists-sync": "0.0.4",
     "najax": "^1.0.2",
+    "resolve": "^1.8.1",
     "rsvp": "^4.7.0",
     "simple-dom": "^1.0.0",
     "source-map-support": "^0.5.0"

src/ember-app.js

@@ -6,7 +6,7 @@ const chalk = require('chalk');
 
 const najax = require('najax');
 const SimpleDOM = require('simple-dom');
-const existsSync = require('exists-sync');
+const resolve = require('resolve');
 const debug = require('debug')('fastboot:ember-app');
 
 const FastBootInfo = require('./fastboot-info');
@@ -119,16 +119,12 @@ class EmberApp {
       debug("module whitelisted; module=%s", whitelistedModule);
     });
 
-    return function(moduleName) {
-      if (whitelist.indexOf(moduleName) > -1) {
-        let nodeModulesPath = path.join(distPath, 'node_modules', moduleName);
+    return function(modulePath) {
+      let moduleName = modulePath.split('/')[0];
 
-        if (existsSync(nodeModulesPath)) {
-          return require(nodeModulesPath);
-        } else {
-          // If it's not on disk, assume it's a built-in node package
-          return require(moduleName);
-        }
+      if (whitelist.indexOf(moduleName) > -1) {
+        let nodeModulesPath = resolve.sync(modulePath, { basedir: distPath });
+        return require(nodeModulesPath);
       } else {
         throw new Error("Unable to require module '" + moduleName + "' because it was not in the whitelist.");
       }

test/fixtures/custom-sandbox/custom-sandbox.js

@@ -2,7 +2,6 @@
 
 const vm = require('vm');
 const fs = require('fs');
-const existsSync = require('exists-sync');
 const Sandbox = require('./../../../src/sandbox');
 
 /**