MediaWiki has several classes that are responsible for handling the user options, namely UserOptionsLookup, UserOptionsManager and DefaultOptionsLookup. All of those classes are currently in the MediaWiki\User namespace, making them a bit lost with all the user-related classes.
Since we're adding a new class for conditional user defaults as part of T321527: Support conditional defaults for user properties, let's make the classes to the (already existing) MediaWiki\User\Options namespace, to make the code a bit more organised. Splitting this process to a new task, since it involves a lot of patches (actually renaming in core and changing the class names in all extensions/skins that make use of it).