Fixes T8547. I wasn't immediately able to reproduce this locally (although I didn't try too hard), but I think the issue is that when atoms extend ghosts (probably they are usually ghosts themselves?), we try to check the ghost language and fatal.
Instead, don't match ghosts when figuring out what an atom extends.
This could maybe be a little cleaner (match the ghosts, at lower priority, and show that they're ghosts?) but I'm not sure there's a real product use case for it, and this looks like a safer way to stop the bleeding for now.