Issue #2917883 by quietone, TR, Jo Fitzgerald, masipila, phenaproxima: Rename 'migration_templates' directories in core modules to 'migrations'

Issue #2569805 by quietone, Jo Fitzgerald, mikeryan, joelpittet, heddn, Adita, Miguel.kode, iMiksu, tvb, maxocub, juancasantito, phenaproxima, alexpott, neclimdul, xjm: For Drupal migration, identify the source module

Issue #2598376 by quietone: d6_user_settings migration user_register constants don't seem to line up

Issue #2534042 by mikeryan, phenaproxima: Move module-specific migration support into the user module

Issue #2463909 by phenaproxima, mikeryan: Migrations should support non-installed default configurations (templates)

Issue #2460529 by alexpott, chx, phenaproxima: Migrations need to use the configuration entity dependency system

Issue #2466797 by mikeryan, benjy: Rename migration_groups key to avoid confusion with MigrationGroup support