title: Composer update gives error saying plugin could not be initialized, class not found for every package
body: <p>I've loaded up a vagrant box from scratch with an existing codebase that has been running on several machines for quite a while. When running composer update it always gives a similar issue:</p>
  
  <pre><code>  - Removing kylekatarnls/update-helper (1.1.1)
  
  
    [UnexpectedValueException]
    Plugin kylekatarnls/update-helper could not be initialized, class not found: UpdateHelper\ComposerPlugin
  </code></pre>
  
  <p>php -v:</p>
  
  <pre><code>Copyright (c) 1997-2018 The PHP Group
  Zend Engine v3.1.0, Copyright (c) 1998-2018 Zend Technologies
      with Zend OPcache v7.1.30-1+ubuntu16.04.1+deb.sury.org+1, Copyright (c) 1999-2018, by Zend Technologies
      with Xdebug v2.7.1, Copyright (c) 2002-2019, by Derick Rethans
  </code></pre>
  
  <p>Composer version 1.8.6</p>
  
  <p>I've tried changing versions of vagrant, virtualbox, php, xdebug, composer all to match versions running on other machines. I've tried composer clearcache, composer dump-autoload, various different composer update methods. Have tried deleting vendor folder and doing composer install</p>