Add module scope plugin (#2189)

* Add module scope plugin

* Oops

* Add comments

* Check windows seps too

* More descriptive error

* Document it

packages/react-dev-utils/ModuleScopePlugin.js

@@ -0,0 +1,68 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+'use strict';
+
+const chalk = require('chalk');
+const path = require('path');
+
+class ModuleScopePlugin {
+  constructor(appSrc) {
+    this.appSrc = appSrc;
+  }
+
+  apply(resolver) {
+    const { appSrc } = this;
+    resolver.plugin('file', (request, callback) => {
+      // Unknown issuer, probably webpack internals
+      if (!request.context.issuer) {
+        return callback();
+      }
+      if (
+        // If this resolves to a node_module, we don't care what happens next
+        request.descriptionFileRoot.indexOf('/node_modules/') !== -1 ||
+        request.descriptionFileRoot.indexOf('\\node_modules\\') !== -1 ||
+        // Make sure this request was manual
+        !request.__innerRequest_request
+      ) {
+        return callback();
+      }
+      // Resolve the issuer from our appSrc and make sure it's one of our files
+      // Maybe an indexOf === 0 would be better?
+      const relative = path.relative(appSrc, request.context.issuer);
+      // If it's not in src/ or a subdirectory, not our request!
+      if (relative[0] === '.') {
+        return callback();
+      }
+      // Find path from src to the requested file
+      const requestRelative = path.relative(
+        appSrc,
+        path.resolve(
+          path.dirname(request.context.issuer),
+          request.__innerRequest_request
+        )
+      );
+      // Error if in a parent directory of src/
+      if (requestRelative[0] === '.') {
+        callback(
+          new Error(
+            `You attempted to import ${chalk.cyan(request.__innerRequest_request)} which falls outside of the project ${chalk.cyan('src/')} directory. ` +
+              `Relative imports outside of ${chalk.cyan('src/')} are not supported. ` +
+              `You can either move it inside ${chalk.cyan('src/')}, or add a symlink to it from project's ${chalk.cyan('node_modules/')}.`
+          ),
+          request
+        );
+      } else {
+        callback();
+      }
+    });
+  }
+}
+
+module.exports = ModuleScopePlugin;