R1:70f6bf306f24

Implement child/descendant query rules in Projects

Summary:
Ref T10010. This adds infrastructure for querying projects by type, depth, parent and ancestor.

I needed to revise the "extended policy check" cycle detection rules. When, e.g., querying a grandchild, they incorrectly detected a cycle because both the child and grandchild needed to check the policy of the grandparent.

Instead, simplify it to just do a basic runaway calldepth check. There are many other safety mechanisms to make it so this can't…