Fix "Could not scan for classes inside "/var/www/$PROJECT/web/core/lib/Drupal.php" which does not appear to be a file nor a folder."

This originally appeared on

composer install

after adding patterns to .gitignore and removing files from git using

git rm -r --cached <pattern>

To fix it, use

rm -rf <pattern>

on all removed files, then do

composer install

again.