sql: operate on pointers to PhysicalPlan

This commit changes `createPlanFor*` methods to operate on pointers to
`PhysicalPlan` instead of values. It also renames and cleans up some
explain-related utility methods.

Release note: None

pkg/sql/create_stats.go

@@ -348,14 +348,14 @@ func makeColStatKey(cols []sqlbase.ColumnID) string {
  return colSet.String()
 }
 
-// makePlanForExplainDistSQL is part of the distSQLExplainable interface.
-func (n *createStatsNode) makePlanForExplainDistSQL(
+// newPlanForExplainDistSQL is part of the distSQLExplainable interface.
+func (n *createStatsNode) newPlanForExplainDistSQL(
  planCtx *PlanningCtx, distSQLPlanner *DistSQLPlanner,
-) (PhysicalPlan, error) {
+) (*PhysicalPlan, error) {
  // Create a job record but don't actually start the job.
  record, err := n.makeJobRecord(planCtx.ctx)
  if err != nil {
- return PhysicalPlan{}, err
+ return nil, err
  }
  job := n.p.ExecCfg().JobRegistry.NewJob(*record)