Isolate TaskRun↔Pod translation into a separate package

[imjasonh]

Expected Behavior

TaskRun→Pod and PodStatus→TaskRunStatus translation code should be organized into its own package, ideally with minimal exported symbols.

This would make it easier to test and use in isolation: the TaskRun reconciler would simply be able to call pod.MakePod(taskRun) and create the correct Pod for the TaskRun, without being exposed to any details of what that even entails.

Actual Behavior

MakePod is buried among dozens of exported methods in pkg/reconciler/taskrun/resources, which also contains code for PipelineResources.

TaskRun→Pod translation also includes entrypoint rewriting and copy step injection, which has leaked into taskrun.go.

PodStatus→TaskRunStatus translation is in a separate package, in status.UpdateStatusFromPod, which happens to unnecessarily take a kubeclient.

/assign
/kind cleanup