Proper scoping for vdbg! + consistent output ordering (#4900)
### Description
1. You currently can't do `turbo_tasks::vdbg!` as the macro expects
itself to be imported.
2. `vdbg!(a, b)` might print a and b out of order, and any other output
from other threads might also be interleaved in. With this patch, a and
b will always show up one after the other.
