title: Why webpack addes a './' to my extern library?
body: <p>I am using webpack to build both dev and prod code. I added an extern lib correctly in webpack.config.js as below:</p>
  
  <pre><code>externals: {
      dagreD3: 'dagre-d3',
  }
  </code></pre>
  
  <p>Of course, I also added the src link to index.html correctly as below:</p>
  
  <pre><code>&lt;script type="text/javascript" src="https://dagrejs.github.io/project/dagre-d3/latest/dagre-d3.min.js"&gt;&lt;/script&gt;
  </code></pre>
  
  <p>The dev server works fine and I can use the library without problem. But when I build the prod server, for some reason, a './' was appended to the head of the 'src'. So the source become this:</p>
  
  <pre><code>src="./https://dagrejs.github.io/project/dagre-d3/latest/dagre-d3.min.js"
  </code></pre>
  
  <p>This breaks the server. I wonder why it is different. Both webpack.prod.config.js and webpack.dev.config.js are loading the same config with the extern:</p>
  
  <pre><code>let baseWebpackConfig = require('./webpack.config');
  </code></pre>
  
  <p>Thank you so much ~!</p>