Issue #2427335 by benjy, chx, dawehner: Combine legacy Source class into SourcePluginBase

core/modules/migrate/src/MigrateExecutable.php

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

...modules/migrate/src/Plugin/migrate/source/EmptySource.php

...es/migrate/src/Plugin/migrate/source/SourcePluginBase.php

core/modules/migrate/src/Plugin/migrate/source/SqlBase.php

core/modules/migrate/src/Row.php

core/modules/migrate/src/Source.php

.../modules/migrate/tests/src/Unit/MigrateExecutableTest.php

core/modules/migrate/tests/src/Unit/MigrateSourceTest.php

...dules/migrate/tests/src/Unit/MigrateSqlSourceTestCase.php

.../modules/migrate/tests/src/Unit/TestMigrateExecutable.php

...les/migrate_drupal/src/Plugin/migrate/source/Variable.php

...les/migrate_drupal/src/Plugin/migrate/source/d6/Block.php

...e_drupal/src/Plugin/migrate/source/d6/CommentVariable.php

...e_drupal/src/Plugin/migrate/source/d6/ContactSettings.php

.../Plugin/migrate/source/d6/FieldInstancePerFormDisplay.php

...src/Plugin/migrate/source/d6/FieldInstancePerViewMode.php

...ules/migrate_drupal/src/Plugin/migrate/source/d6/File.php

...ules/migrate_drupal/src/Plugin/migrate/source/d6/Node.php

.../migrate_drupal/src/Plugin/migrate/source/d6/NodeType.php