Drupal 8 - PHP Fatal error: require_once(): Failed opening required 'tcpdf/tcpdf.php'

Created by: demeritcowboy

@KarinG For the issue that came up on the other core PR - if I just make this change in functions.php it seems to get past it:

-require_once 'tcpdf/tcpdf.php';
+new TCPDF();

But then there's a new problem - the settings page gives a 404.