SwiftMailer depends on the mailsystem module.

@wellme, your patch contains an extra info.
I have prepared correct patch

Might as well add support for installing SwiftMailer through Composer using Drupal Packagist.

#5 + #8 in one patch.

I makes sense to add mailsystem as a dependency, it makes it much easier to configure the module because core does not provide a UI for its mail settings and it provides an option to use a specific theme for every email. Which is important for swiftmailer since twig uses templates for all emails.

We use mailsystem in all our projects and recommend to use it when using simplenews, so +1.

I see you didn't add the composer dependency, I personally don't care, but people have very different opinions about that.

I think composer is for external dependencies. And the Packagist implementation will handle it automatically.

Released 8.x-1.0-alpha2 with the new dependency. This should fix the issues with Drupal CI. Thanks all!