Linking library symbol to actionscript + package name?

I’m trying to export a library item to actionscript. However, I want to have the .as file under a package (say, engine.LibraryItem).

The only to get Flash to understand the location of the source file is to put the full name (engine.LibraryItem) in the class name under the library item options for exporting. However, when I do this, the base class disappears (which normally is MovieClip), and Flash gives an error about my LibraryItem source not extending MovieClip (even though it does in the actual source). How do I get around this??