Clean up ./pkg/repo

[arschles]

Now that #291 is in, all of the code in ./pkg/repo has been refactored for use in #290 and moved to ./pkg/module. This issue is for removing all of ./pkg/repo.

A lot of code up the stack relies on ./pkg/repo, so of course that code will need to be removed as well (see #312). Dependent code goes all the way up to asynchronous workers (also part of #290) and the proxy fetch handler, so all of that will need to be removed at once. We don't need to do all of that before we can consider #290 done, but this cleanup will get rid of a lot of code and will be 🎉

We should also use this opportunity to remove the gomod parser since I think the only places that use it are the aforementioned fetch route (if I'm wrong, let's keep it)

cc/ @marwan-at-work @robjloranger @marpio since we've all talked about this recently

[arschles]

I'll take this one

(after #290 is done)