Resolve the rough edges around the mapper/mapping package DX flow.

[jsimnz]

As mentioned a few times in #471 the DevEx flow of using the mapper package within the query planner and elsewhere, as well as its relation to the parser package, needs to be refined.

This is a tracking issue for that effort. It doesn't need to be done all at once, but we can

1. Identify the concrete instances
2. Address "low-hanging" instances
3. Continue to make more core refactor changes as necessary.

Child issues:

• Revisit Select case in planner.newPlan #566

[fredcarle]

Stale