* fix(steam): share the Steam app's prefix with shortcuts of the same install
* fix(steam): prefer the matching install's save prefix
* fix(steam): tolerate an unreadable library when linking a shortcut prefix
A library folder that cannot be listed made the shortcut writer exit, so no added game was registered.
* fix(steam): repair a dangling shortcut prefix link and keep Steam library installs to their own prefix
* fix(steam): only change shortcut prefix links the importer created
* fix(steam): record a prefix link only when the importer created it