What happened to Preview's "Markup.appex"? There's a Markup app extension in Mojave--it's clearly installed, because I can use it. Where did it go, though? It used to be in /Applications/Preview.app/Contents/Plugins/ in past versions of macOS, but it's not there anymore.
Right-clicking on the Markup extension in System Preferences > Extensions offers a "show in Finder" option, which yields:

/System/Library/CoreServices/Finder.app/Contents/PlugIns/PlaceholderMarkUp.appex