ExternalProject: Avoid reconfigure when updates are disconnected
When UPDATE_DISCONNECTED is true, create separate update_disconnected and patch_disconnected targets which have ALWAYS set to false. Make the configure step depend on patch_disconnected in this case too. This ensures the configure, build, install and test steps are not executed unnecessarily when updates are disconnected.
Make the update and patch commands depend on the details of those steps. This ensures they are re-executed when any of those details change, even if updates are disconnected.
Allow updates to occur even if UPDATE_DISCONNECTED is true, but don't contact the remote in that case. If asked to update to a ref that isn't known locally, that is now detected and causes a fatal error when updates are disconnected. Previously, the build would have silently and erroneously continued to use the old ref.