FieldItemBase::setValue() relies on code order

Problem/Motivation

FieldItemBase::setValue() depends on the property definition order in some cases, for example when $entity->fieldname = 'somevalue'; is used or when setValue is called with a scalar. The migration EntityContentBase::updateEntity method often does the latter.

Steps to reproduce

Edit DateTimeItem::propertyDefinitions and swap the definition of the two properties and watch DateTimeItemTest fail.

Proposed resolution

Just call mainPropertyName(). As far as I can tell, the main property name was introduced in #2015687: Convert field type to FieldType plugin for taxonomy module while using the "first property" notion was added a year before that in #1696640: Implement API to unify entity properties and fields and it was simply never updated.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet