Add module scope plugin (#2189)

* Add module scope plugin

* Oops

* Add comments

* Check windows seps too

* More descriptive error

* Document it

packages/react-scripts/config/webpack.config.dev.js

@@ -18,6 +18,7 @@ const CaseSensitivePathsPlugin = require('case-sensitive-paths-webpack-plugin');
 const InterpolateHtmlPlugin = require('react-dev-utils/InterpolateHtmlPlugin');
 const WatchMissingNodeModulesPlugin = require('react-dev-utils/WatchMissingNodeModulesPlugin');
 const eslintFormatter = require('react-dev-utils/eslintFormatter');
+const ModuleScopePlugin = require('react-dev-utils/ModuleScopePlugin');
 const getClientEnvironment = require('./env');
 const paths = require('./paths');
 
@@ -106,6 +107,14 @@ module.exports = {
       // https://www.smashingmagazine.com/2016/08/a-glimpse-into-the-future-with-react-native-for-web/
       'react-native': 'react-native-web',
     },
+    plugins: [
+      // Prevents users from importing files from outside of src/ (or node_modules/).
+      // This often causes confusion because we only process files within src/ with babel.
+      // To fix this, we prevent you from importing files out of src/ -- if you'd like to,
+      // please link the files into your node_modules/ and let module-resolution kick in.
+      // Make sure your source files are compiled, as they will not be processed in any way.
+      new ModuleScopePlugin(paths.appSrc),
+    ],
   },
   module: {
     strictExportPresence: true,