Issue #2315237 by andypost, daggerhart | yched: Rename FieldDefinition to BaseFieldDefinition.

core/lib/Drupal/Core/Entity/ContentEntityInterface.php

core/lib/Drupal/Core/Entity/EntityManager.php

core/lib/Drupal/Core/Field/BaseFieldDefinition.php

core/lib/Drupal/Core/Field/FieldDefinitionInterface.php

core/lib/Drupal/Core/Field/FieldItemInterface.php

...l/Core/Field/Plugin/DataType/Deriver/FieldItemDeriver.php

...b/Drupal/Core/Field/TypedData/FieldItemDataDefinition.php

core/modules/aggregator/src/Entity/Feed.php

core/modules/aggregator/src/Entity/Item.php

core/modules/block_content/src/Entity/BlockContent.php

core/modules/comment/src/Entity/Comment.php

core/modules/contact/src/Entity/Message.php

.../modules/contact_storage_test/contact_storage_test.module

core/modules/field/src/Entity/FieldInstanceConfig.php

core/modules/field/src/Plugin/views/field/Field.php

core/modules/field/src/Tests/TestItemTest.php

core/modules/file/src/Entity/File.php

.../modules/menu_link_content/src/Entity/MenuLinkContent.php

core/modules/migrate/src/Plugin/migrate/id_map/Sql.php

core/modules/node/src/Entity/Node.php