[wallpaper] Avoid using pluginId for indexing package indexes
Image takes a string and loads the relevant package at a given path wallpapers+packageName
pluginId typically is the same as the package name on disk, but not always, either due to bad metadata or it being deliberately mangled during install to avoid escaped characters.
Returning the package path has the same desired effect for using packages and it resolves correctly.
metaData presence is still queried to separate real packages from the fake packages generated internally which represent just one image.