[Part 2] Single file todos - removes file path hashes

[scalvert]

The previous implementation used file path hashes to identify todos in a map. This was convenient as we'd use the file path hash when generating batches and subsequently manipulating the files on disc as the hash corresponded directly to the directory/filepath mapping from map to physical location. Since we moved to a single file approach, we now no longer need to use hashes, but rely exclusively on the todo itself.

Depends on: #299