I am not sure if the bug is the URL form (SSH and not HTTPS) or service (something needs to index on BitBucket and take time?) but referencing GitHub repos in the form http://github.com/doctrine/common.git works.
Thanks for reporting this.
Issue #4054 was marked as a duplicate of this issue.
Issue #3982 was marked as a duplicate of this issue.
Still experiencing it for submodule url format exactly as described.
This issue has been closed due to inactivity. If you continue to see problems, please reopen or create a new issue.