Add path conflict checks

Previously there were no checks in place to ensure that base and
virtual documents do not overlap. As a result, if users loaded raw
JSON and rules into OPA that overlapped, the evaluation results were
not well defined. With these changes, we can detect the overlap and
reject updates (to policies or data) that would cause inconsistent
results.

Fixes #1207

Signed-off-by: Torin Sandall <torinsandall@gmail.com>