Add the current goal to the initial conflict set while backjumping.
This is relevant for unknown packages. Without the current goal in the initial conflict set, the unknown package itself does not end up in its own conflict set. But we want it to, if only to make sure that the error slicing machinery keeps the message about the unknown goal around. It is also the correct thing to do. The initial conflict set corresponds to a virtual "avoid" choice underneath the package goal. That choice corresponds to avoiding assigning any proper value to the goal at all, which might be possible if the goal was not needed. The conflict set for this "avoid" goal is always between the current package and its immediate goal reason. I have added comments to the code explaining this as well.