[gold] Refactor skiacorrectness.go.
[gold] Refactor skiacorrectness.go.

This non-functional change extracts the logic to instantiate each Gold component into separate, independent functions that clearly state the dependencies between components. Please see the attached bug for the rationale.

The mustMakeRootRouter() function is intentionally kept as one single, big function, even though it could be broken down into smaller functions e.g. on a per-router basis. Given the amount of repetition in the routing code (e.g. versioned vs. unversioned), any changes to this code are probably best left for a separate CL.

Bug: skia:10816
Change-Id: I94bffe4bdefaf8debf9cea644238e40680b08a31
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/323786
Commit-Queue: Leandro Lovisolo <f558143c2568be8d55575d13f30a1fc5cb8009d5@google.com>
Reviewed-by: Kevin Lubick <7cdab2cfab351f23814786ba39716e90eed69047@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot