My understanding is that, by loading only one module at this point, we're safe from overriding here.
- Kohana::modules() call registers three paths (APPPATH, purifier, SYSPATH, in that priority order).
- first Purifier call cues PHP to call the autoloader, which looks in APPPATH (no match) and then purifier (match).
- Purifier class is loaded.

Afterward, the autoloader will never again be called on Purifier calls.  And, if someone tries to manually include their own Purifier class, they'll get a class name conflict.