R1:6cb2bde48d41

Add "Dominion" rules for Owners Packages

Summary:
Ref T10939. This supports two settings for packages (although they can't be configured yet):

- **Strong Dominion**: If the package owns `a/`, it always owns every subpath, even if another package also owns the subpath. For example, if I own `src/differential/`, I always own it even if someone else claims `src/differential/js/` as part of the "Javascript" package. This is the current behavior, and the default.
- **Weak Dominion**: If the package owns…