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 non-generated file get no packages owning it.
  • Normally: lots of packages owning it).
• Created a revision affecting only generated files.
  • When I faked things, saw no Owners actions trigger.
  • Normally: some packages added reviewers or subscribers.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13130

Differential Revision: https://secure.phabricator.com/D19426