Global Import Context Manager

[insertinterestingnamehere]

With VECs, we should provide a context manager that allows users to do global imports instead of requiring that they use the mark_module_as_global interface. Following dependencies like this will hopefully make user-level code less error-prone. As it is currently it's easy to accidentally forget a dependency and end up with it getting included in a VEC unintentionally. Having a global module that isn't aware of VECs depend on a stateful module that has been loaded into a VEC will likely lead to confusing bugs.