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