11 files declare their use of ProcessorInterface in Search API 8

AggregatedFieldsTest.php in tests/src/Unit/Processor/AggregatedFieldsTest.php

ConfigOverrideKernelTest.php in tests/src/Kernel/ConfigEntity/ConfigOverrideKernelTest.php

FieldsHelper.php in src/Utility/FieldsHelper.php

HighlightTest.php in tests/src/Unit/Processor/HighlightTest.php

Index.php in src/Entity/Index.php

IndexInterface.php in src/IndexInterface.php

IndexProcessorsForm.php in src/Form/IndexProcessorsForm.php

Item.php in src/Item/Item.php

ProcessorIntegrationTest.php in tests/src/Functional/ProcessorIntegrationTest.php

UnsavedIndexConfiguration.php in src/UnsavedIndexConfiguration.php

ViewsPropertyExtractionTest.php in tests/src/Kernel/Views/ViewsPropertyExtractionTest.php