Issue #2676258 by benjy, alexpott: Nuke builder plugins and migration storage

core/modules/migrate/migrate.services.yml

core/modules/migrate/src/MigrateTemplateStorage.php

core/modules/migrate/src/MigrateTemplateStorageInterface.php

core/modules/migrate/src/MigrationBuilder.php

core/modules/migrate/src/MigrationBuilderInterface.php

core/modules/migrate/src/Plugin/MigrateBuilderInterface.php

...odules/migrate/src/Plugin/migrate/builder/BuilderBase.php

core/modules/migrate/src/Tests/TemplateTest.php

.../migrate_drupal/src/Plugin/migrate/builder/CckBuilder.php

...ate_drupal/src/Plugin/migrate/builder/d6/CckMigration.php

...odules/migrate_drupal/src/Tests/MigrateDrupalTestBase.php

...es/migrate_drupal/src/Tests/d6/MigrateDrupal6TestBase.php

...es/migrate_drupal/src/Tests/d7/MigrateDrupal7TestBase.php

core/modules/node/src/Plugin/migrate/builder/d6/Node.php

core/modules/node/src/Plugin/migrate/builder/d7/Node.php

...ules/node/src/Tests/Migrate/d7/NodeMigrateDeriverTest.php

...dules/taxonomy/src/Plugin/migrate/builder/d6/TermNode.php

core/modules/user/src/Plugin/migrate/User.php

...ules/user/src/Plugin/migrate/builder/d6/ProfileValues.php

core/modules/user/src/Plugin/migrate/builder/d7/User.php