We have a mix of upper and lower cased folders in MediaWiki core, especially in includes.
Having this mix does not help towards PSR-4 related goals.
It's going to be a little painful, but I think we should JFDI and rename the lot in one go...
See also: T353458: Make core classes follow PSR-4, T278278: Adopt PSR-4 namespaces in WMF-deployed extensions/skins.