title: `Cannot read Component` in a React Component npm module
body: <p>I have a small set of reusable react components which i am attempting to bundle together into a module.</p>
  
  <p>However when I include them in another product and try to build I get <code>Cannot read property 'Component' of undefined</code>. Webpack seems to not have a reference to <code>_react.Component</code> yet <code>_react</code> does exist.</p>
  
  <p>My components are compiled into a <code>/dist</code> directory using the following config:</p>
  
  <pre><code>var config = {
      entry: path.resolve('./src/index.jsx'),
      devtool: 'source-map',
      output: {
          path: path.resolve('./dist'),
          filename: 'react-components.js'
      },
      externals: {
          react: {
              root: 'React',
              commonjs2: 'react',
              commonjs: 'react',
              amd: 'react',
          },
          'react-dom': {
              root: 'ReactDOM',
              commonjs2: 'react-dom',
              commonjs: 'react-dom',
              amd: 'react-dom',
          },
          'react-bootstrap': {
              root: 'ReactBootstrap',
              commonjs2: 'react-bootstrap',
              commonjs: 'react-bootstrap',
              amd: 'react-bootstrap',
          }
      },
      module: {
          loaders: [
              {
                  test: /(\.jsx|\.js)$/,
                  loader: 'babel',
                  exclude: /(node_modules|bower_components)/
              }
          ]
      }
  };
  </code></pre>
  
  <p>I then include them in my other project using <code>import { Component1, Component2 } from 'react-components'</code>.</p>
  
  <p>My webpack config for the project I include them in is pretty much the same as above, i've listed the differences below:</p>
  
  <pre><code>var config = {
      entry: path.resolve('./src/example.jsx'),
      devtool: 'source-map',
      output: {
          path: path.resolve('./src/'),
          filename: 'app.js',
          publicPath: 'http://localhost:8080/src',
      },
      devServer: {
          hot: true,
          inline: true,
          port: 8080,
          historyApiFallback: true,
      },
      externals: {
          'cheerio': 'window',
          'react/lib/ExecutionEnvironment': true,
          'react/lib/ReactContext': true,
      },
      module: {
          loaders: [
              {
                  test: /(\.jsx|\.js)$/,
                  loader: 'babel',
                  exclude: /(node_modules|bower_components)/,
                  presets: [
                      'airbnb'
                  ]
              }
          ]
      }
  };
  </code></pre>