Issue #2302021 by swentel, yched, undertext: Move options.module widgets in Core.

...e/Field/Plugin/Field/FieldWidget/OptionsButtonsWidget.php

...re/Field/Plugin/Field/FieldWidget/OptionsSelectWidget.php

...Core/Field/Plugin/Field/FieldWidget/OptionsWidgetBase.php

...erence/src/Tests/EntityReferenceFieldDefaultValueTest.php

...ty_reference/src/Tests/EntityReferenceIntegrationTest.php

...es/entity_reference/src/Tests/EntityReferenceItemTest.php

...rence/src/Tests/Views/EntityReferenceRelationshipTest.php

core/modules/rdf/src/Tests/Field/EntityReferenceRdfaTest.php

...les/rdf/src/Tests/Field/TaxonomyTermReferenceRdfaTest.php

...s/system/src/Tests/Entity/EntityQueryRelationshipTest.php

...ules/taxonomy/src/Tests/TaxonomyTermReferenceItemTest.php

...ules/taxonomy/src/Tests/Views/TaxonomyFieldFilterTest.php

core/modules/taxonomy/taxonomy.info.yml