title: composer install -n --ignore-platform-reqs not ignoring PHP extension
body: <p>we have circle build that runs <code>composer install -n --ignore-platform-reqs --no-dev</code> but this is not ignoring the platform requirements anymore. </p>
  
  <p>This is what i see in the circle log. The <code>--ignore-platform-reqs</code> is clearly not working. Any ideas why please?</p>
  
  <pre><code>Your requirements could not be resolved to an installable set of packages.
  
    Problem 1
      - Installation request for drupal/core 8.6.13 -&gt; satisfiable by drupal/core[8.6.13].
      - drupal/core 8.6.13 requires ext-pdo * -&gt; the requested PHP extension pdo is missing from your system.
    Problem 2
      - typo3/phar-stream-wrapper v2.1.0 requires ext-fileinfo * -&gt; the requested PHP extension fileinfo is missing from your system.
      - typo3/phar-stream-wrapper v2.1.0 requires ext-fileinfo * -&gt; the requested PHP extension fileinfo is missing from your system.
      - Installation request for typo3/phar-stream-wrapper v2.1.0 -&gt; satisfiable by typo3/phar-stream-wrapper[v2.1.0].
  </code></pre>