Run Mixin configuration explicitly at a better and well defined time

[Player3324]

This PR shows how we can directly invoke Mixin's configuration phase that goes through all the configs and potentially plugins. Normally this runs a bit later, when the first class gets loaded, but at that point we lost control over the class path restrictions and conflate any errors with other activity.

Resolves issues like #998 or when Mixin plugins end up loading MC classes before they should.

[LlamaLad7]

Seems like quite a lot of digging into mixin internals when you could just ask mixin to transform a dummy class?

[Player3324]

I'm not too concerned about that, might add some actual api for this eventually. Using a dummy class is not a good workaround btw, it'd involve a lot more complexity than this, which is really just lock.push(); checkSelect(); lock.pop().