Swiftmailer module installation fails although the swiftmailer external library is installed
That's 7.x/outdated information unfortunately.

You should use composer to manage your Drupal 8 site, using the drupal-project starting point: https://github.com/drupal-composer/drupal-project. It will be a bit of work to switch an existing site to that but after that, installing swiftmailer including all its dependencies will be as easy as running composer require drupal/swiftmailer. That will automatically include all dependencies.