rego: Plumb the newer QueryTracer through Rego API's

This will deprecate the older API's that used the `topdown.Tracer` in
favor of the newer `topdown.QueryTracer` interface. Usages of the old
API have been swapped, although some of the testing is left with them
to ensure we still support them (until we remove the deprecated API).

Signed-off-by: Patrick East <east.patrick@gmail.com>