v2.7.1336

Optimize contest loading solely from event feed file

When a disk contest source is pointing directly at an event feed file (e.g. running
the resolver pointing at only an ndjson file, no contest archive), there is no point
looking for local file references or caching the set of file references we've found
on disk, because they don't exist.

Putting in these three checks saves ~600ms loading a reasonably large contest on my M4,
but likely saves several seconds on older hardware, and way more on a non-SSD.

Part of #1129.