R1:5e2af4b9b5cd

Prepare to support an "Ignore generated files" flag in Owners

Summary:
Depends on D19425. Ref T13130. See PHI251. Now that changesets have a durable "generated" attribute, we can let owners packages check it when we're computing which packages are affected by a revision.

There's no way to actualy configure a package to have this behavior yet.

Test Plan:
- Created a revision affecting a generated file and a non-generated file.
- When I faked `mustMatchUngeneratedPaths()` to `return true;`, saw the…