https://www.mediawiki.org/wiki/Best_practices_for_extensions#File_structure
SHOULD: Classes in MediaWiki\Extension\<ExtensionName> namespace. MediaWiki\<ExtensionName> is permissible if the extension name is a sufficiently unique word and not something generic (e.g. not a verb or noun).
For reasons (some of which will be my fault), numerous extensions are in MediaWiki\Extensions\<ExtensionName> rather than MediaWiki\Extension\<ExtensionName>
We should eventually clean these up. OAuth, ReadingLists and SecurePoll are some of the biggest ones. The other smaller ones should be easier (but may need some temporary aliasing).