Missing features to port Angular monorepo

[alexeagle]

I'd love to use this for Angular:

https://github.com/angular/angular/blob/master/.github/CODEOWNERS which you can see DESPERATELY needs some help to scale

Here's a start: https://github.com/angular/angular/compare/master...alexeagle:codeowners we can only make the switch when the new test passes, currently the delta is https://gist.github.com/alexeagle/d6bf98f1c36923a77caa220edde00063

What we learn from this is:

1. packages can have multiple rules. Simple motivating example: maybe I want my Bazel expert to review BUILD.bazel file but anyone can review other files
2. Angular uses @angular/framework-global-approvers as a "global" approver and that should be expressible in the root of the tree, not repeated in each package