Create a Sphinx directive that automatically discovered and lists the tasks that call a given task. This will be used in task documentation to provide two-way linking up and down the task call stack (the subtask listing provides linking down the call stack).

I think there should be two types of links, treated slightly differently:

• Call from regular Tasks should be direct parent-child relationship.
• Any CmdLineTask that ultimately uses a task should be listed.

Note that the language needs to be hedged as "default" users since subtask retargeting entirely changes the call pattern. This can't be documented with Sphinx until we build a dynamic task documentation app.