Change course_id to package_id

on new Locators to make it clear that they have a different syntax
than Location.course_id altho they represent the same info.

I used the term 'package' as the id is a name for identifying a collection of xblocks which do not necessarily need to be a course (and the syntax looks like a package in the programming sense)

I went ahead and changed the split persistence representation since it's never been live. I need to add a story to change the 'course_id' property in location_map to 'package_id' via a db migration mechanism.

@singingwolfboy @andy-armstrong Please review.