This removes the funky auto-injection capability from migration
closures. While technically removing a feature, this means we do
not need a fully-wired IoC container e.g. during installation.
Instead, all migration closures simply receive a schema builder
object (which is what most of them were already doing anyway).
This will make it much easier for extension developers (and also less
error-prone) to create migrations for things like creating tables,
renaming columns and so on...
