In most cases, we assume any connection is insecure unless the URL
starts with "https://", "$USERNAME@", or "ssh". There are a few
exceptions: I'm assuming all Emacswiki packages are insecure, and I
don't think we can know whether packages installed via Google Go are
secure or not.
Since emacswiki recipes might not have an explicit :url in their recipe,
the :compute-checksum method should fallback to the base url, like the
:install and :guess-website methods.
Currently this has no function other than documentation, but it could
be given a function later, such as allowing manually-written recipes
to take precedence over auto-generated ones.
This ensures that they will pick up updates to their parent methods
without having to add any code. In particular, git-svn and emacsmirror
will automatically pick up the new git checksum method.