title: How to concat files with rollup?
body: <p>I have the following files:</p>
  
  <pre><code>// a.js
  var g = {
      version: 1,
      extend: function(source, target) {
          console.log('in g.extend');
      }
  };
  
  // b.js
  g.extend(g, {
      h: function() {
          console.log('in h');
      }
  });
  
  // index.js
  import './a';
  import './b';
  </code></pre>
  
  <p>The result of building(as iife):</p>
  
  <pre><code>(function () {
      'use strict';
  
      var g$1 = {
          version: 1,
          extend: function(source, target) {
              console.log('in g.extend');
          }
      };
  
      g.extend(g, {
          h: function() {
              console.log('in h');
          }
      });
  
  }());
  </code></pre>
  
  <p>Rollup rename the <code>g</code> variable to <code>g$1</code>.</p>
  
  <p>How can I prevent this ?</p>
  
  <p>And is this the correct way to concat files ?</p>
  
  <p>Thanks.</p>