Links to git for luci-go and other github.com infra repos don't work.

To reproduce:
1) Go to https://cs.chromium.org/chromium/infra/go/src/github.com/luci/luci-go/appengine/cmd/helloworld/frontend/handler.go
2) Click "View in" in the top right and click on "git" or "git blame"
3) See sad robot page instead of source.

The destination URL is https://chromium.googlesource.com/infra/infra/+/master/go/src/github.com/luci/luci-go/appengine/cmd/helloworld/frontend/handler.go

when it should be
https://chromium.googlesource.com/external/github.com/luci/luci-go/+/master/appengine/cmd/helloworld/frontend/handler.go