Clean up copy detection code a bit
Summary:
Ref T1266. This doesn't change any behaviors, but some of this code has a lot of really complicated conditionals and I tried to break that up a bit.
Also, reexpress this stuff in terms of the "structured" parser in D12144.
Test Plan: Unit tests still pass. They aren't hugely comprehensive but did reliably fail when I screwed stuff up.
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T1266
Differential Revision: https://secure.phabricator.com/D12145