(new-types-package-org-2) Make builtin mapping stuff a top-level submodule

Summary:
Since this orchestrates between dagster.config and soon-to-be
dagster.core.types, but this at the top-level

Depends on D1821

Test Plan: BK

Reviewers: alangenfeld, max

Reviewed By: alangenfeld

Differential Revision: https://dagster.phacility.com/D1822