codeintel: transitively autoindex go dependencies for any github repo

Created by: shrouxm

Review by commit should work well. Currently this is still hardcoded to only run on sourcegraph/sourcegraph until we can figure out the right way to rate limit.

@unknwon this is mostly just setting things up on the code intel side, starting with Go since it doesn't need any gitserver updates 'cause GitHub is the package manager. Just wanted you to check whether the way I'm using repoupdater in the second commit e861760 looks reasonable.

@efritz there's some more testing/cleanup but i'm assuming you won't read this 'til your long weekend's done right? :monocle_face:

cc @olafurpg for context, the recognizer.EnsurePackageRepo and recognizer.InferPackageIndexJobs are approximately where a lot of the PackageHub logic will end up in the backend.