Refactoring changes

[magnatelee]

There were some loose ends in the C++ API that accidentally exposed Legion names directly to client libraries. This would be confusing to developers, as they might think they need to learn two APIs, even though they would, and should, only need to use a small fraction of the Legion API in Legate in most cases. This PR makes the following refactoring changes to hide Legion references from the Legate API so developers can learn only one API for Legate:

1. New callback API for Legate libraries that passes no Legion handles to them
2. Import and re-export Legion names that we need to expose
3. Use qualified names for all Legion references in the core code not imported in 2. This isn't strictly necessary for the goal, but is good for code hygiene.

The PR also simplifies and hides all the implementation detail of task variant registration as a bonus point.

[manopapad]

One trivial comment, otherwise looks good