[breaking change] Clean up code related to the uses of `CommandTypes.Workflow` and `WorkflowInfo` in `System.Management.Automation`. This change mainly affects help provider code.
This PR includes a few minor breaking changes:
- Change the `public` constructors of `WorkflowInfo` to `internal`. We don't support workflow in PSCore, so I think it makes sense to not allow people to create `Workflow` instances.
- Remove the type `System.Management.Automation.DebugSource` since it's only used for workflow debugging.
- Remove the overload of `SetParent` from the abstract class `Debugger` that is only used for workflow debugging.
- Remove the same overload of `SetParent` from the derived class `RemotingJobDebugger`.