Issue #2315333 by tim.plunkett: Move block plugin code out of block.module.

core/core.services.yml

core/lib/Drupal/Core/Block/Annotation/Block.php

core/lib/Drupal/Core/Block/BlockBase.php

core/lib/Drupal/Core/Block/BlockManager.php

core/lib/Drupal/Core/Block/BlockManagerInterface.php

core/lib/Drupal/Core/Block/BlockPluginInterface.php

core/lib/Drupal/Core/Extension/ThemeHandlerInterface.php

...dules/aggregator/src/Plugin/Block/AggregatorFeedBlock.php

core/modules/block/block.api.php

core/modules/block/block.services.yml

core/modules/block/src/BlockInterface.php

core/modules/block/src/BlockListBuilder.php

core/modules/block/src/BlockPluginBag.php

...s/block/src/Controller/CategoryAutocompleteController.php

core/modules/block/src/Event/BlockConditionContextEvent.php

core/modules/block/src/Event/BlockEvents.php

core/modules/block/src/Tests/BlockConfigSchemaTest.php

core/modules/block/src/Tests/BlockInterfaceTest.php

.../modules/block/tests/modules/block_test/block_test.module

...es/block_test/src/Plugin/Block/TestBlockInstantiation.php