migrate_source data type needs to define the constants mapping

Problem/Motivation

While writing tests for the Drupal 7-to-8 migration path, I've run into a lot of config schema exceptions due to migrations defining source constants. Turns out that migrate's data_types.schema.yml doesn't define the constants mapping as part of the migrate_source data type; adding it solves this problem.

This is a major issue because it blocks almost all child issues of #2456259: [META] Drupal 7 to Drupal 8 Migration path.

Proposed Resolution

Add the constants mapping as an ignored property of the migrate_source data type.

Remaining Tasks

Patch, review, commit.

API/UI Changes

Nil.