Avoid shading mixin packages

[CursedFlames]

This will cause issues with plugins/etc that touch mixin, as only regular mod classes are redirected to the shaded classes.

Ideally we would substitute forge's mixin dependency in such a way that it just transparently replaces the original package and we don't need to do anything more; I don't know if we can get an early enough entrypoint to do this while still having MixinTransmogrifier be a simple jar in the mods folder.

Probably worth discussing with NeoForge devs etc?