Issue #2533984 by phenaproxima: Move module-specific migration support into the aggregator module

...les/aggregator/migration_templates/d6_aggregator_feed.yml

...les/aggregator/migration_templates/d6_aggregator_item.yml

...aggregator/migration_templates/d6_aggregator_settings.yml

...aggregator/migration_templates/d7_aggregator_settings.yml

...ggregator/src/Plugin/migrate/source/d6/AggregatorFeed.php

...ggregator/src/Plugin/migrate/source/d6/AggregatorItem.php

...tor/src/Tests/Migrate/d6/MigrateAggregatorConfigsTest.php

...egator/src/Tests/Migrate/d6/MigrateAggregatorFeedTest.php

...egator/src/Tests/Migrate/d6/MigrateAggregatorItemTest.php

...or/src/Tests/Migrate/d7/MigrateAggregatorSettingsTest.php

.../src/Unit/Plugin/migrate/source/d6/AggregatorFeedTest.php

.../src/Unit/Plugin/migrate/source/d6/AggregatorItemTest.php