Is your feature request related to a problem? Please describe.
During office hours, we discussed testing athens, and it seemed like one issue is that we have to actually call out to somewhere public to run tests. This isn't ideal for several reasons.

Describe the solution you'd like
Ideally, this would just be a module.Fetcher implemented against a set of local filesystem objects

Describe alternatives you've considered
None considered

Additional context
Add any other context or screenshots about the feature request here.