Can no longer run "composer require drupal/swiftmailer"
I know you're sorted now, but wanted to suggest that this could be because the repository URL for the coder module is using https.

See here:

https://help.github.com/articles/why-is-git-always-asking-for-my-password/

The link at the bottom of that article suggests that you can cache your credentials to avoid the need to enter them when using https:

https://help.github.com/articles/caching-your-github-password-in-git/

(I would have commented rather than answered, but it seems I don't have enough reputation to comment yet!)