Users should not be able to select lesson prerequisites outside of current course

### Reproduction Steps

Open a lesson in lesson editor(Not course builder)
Enable prerequisites and try to select a lesson. You'll notice all lessons from different courses show up as well

### Expected Behavior

Meant to show the lessons in the current course as a prerequisite

### Actual Behavior

Shows all lessons in all courses

HS-119847

Reproduce the issue by opening a lesson in the lesson editor, enabling prerequisites, and checking the lesson selector. Trace the selector's data source to determine why lessons from other courses appear. Done means the selector shows only lessons from the current course and still allows valid prerequisites.

Written by the indexing model from the issue text.