VisitAllSourceTypesAndNamespaces parallelizes even when the resulting execution is slower

[sharwell]

During evaluation of feedback High CPU usage when idle, calls to VisitAllSourceTypesAndNamespaces reveal CPU performance overhead of 15% for execution of tasks, while less than 3% of the overall CPU usage is actually used by the body of these tasks. As an initial step to improve this overhead, the VisitAllSourceTypesAndNamespaces method should be updated to avoid wrapping execution in a Task for cases where the source only contains a single type.