title: Can I change the PSR 4 Autoload Namespacing with a fallback?
body: <p>I have a public package on packagist/composer.</p>
  
  <p>Originally it autoloaded with PSR-4 \GitHubUser\Package\Class for the file src/Class.php.</p>
  
  <p>I would like to shorten this to \Package\Class for the same file. This is easily done by changing the composer.json. But is there a way I can do this with a fallback for people using the existing longer call? (autoload both?)</p>
  
  <p>What I'd like:</p>
  
  <pre><code>"autoload": {
      "psr-4": {
          "PackageName\\": "src",
          "User\\OldPackageName\\": "src"
      }
  }
  </code></pre>
  
  <p>But it doesn't register the 2nd call for the same folder.</p>