Upgrading a site that still has "mysql" in the dsn string breaks in latest master
I think it's partly because of https://github.com/civicrm/civicrm-packages/pull/302 but more because I remember once seeing something somewhere that civi silently converted mysql to mysqli for you, and maybe that has also been removed somewhere. While reviewing it maybe I missed it.
The fix is easy, just update your DSN in civicrm.settings.php to explicitly say "mysqli".
The technical issue is that without that silent conversion, it ends up trying to load php-mysql. It's just confusing at first because the error is
extension not found, which makes you think "Civi Extension", but it means php extension.
Going to label it regression, but it's not the usual type of regression and it just started yesterday.