THROWAWAY: Don't load unmodified generated external classes

THROWAWAY: Don't load unmodified generated external classes

Loading external classes is only useful in dynamic mode, or when
transitioning to static mode, and loading code we generated ourselves is
never useful.

However, mst pointed out a better approach on IRC, so this commit is
only so I can pick it up from elsewhere and change it to this approach:

> I would argue that if the installed thing has a Schema::Loader md5 in
> it, you should ignore it, and you should only bring custom code in if
> you're writing new files.

> That should handle the dynamic->static transition case, and then you can
> just output a warning or something if you see an extra file but ignore
> it, with a note of "turn option X on if you really wanted this".