Mock out VCS/go get in order to improve testing speed

[twexler]

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.

[marwan-at-work]

@twexler can you specify which tests exactly should be using a mock? I believe we do have a mock implementation of module.Fetcher but we also test the live-internet-y code for good reasons to make sure things function properly. I can help you look around if you're not sure which tests would you like to mock out. Thanks!

[yolocs]

Seems similar to #1359?

[twexler]

@marwan-at-work I need to rewatch yesterday's office hours to regain context, but as mentioned this came up during that meeting

[twexler]

@yolo3301 it seems fairly similar to that issue, maybe sort of a duplicate?

[arschles]

There could be two levels to this. a Mock VCS could help us isolate the caller / response parsing code from the code that sets up the command and environment. the mock binary in #1359 could validate the code to set up the command/env

[arschles]

Seems similar to #1359?

@yolocs yes, I think this is exactly the same. #1449 should fix both of these issues