While doing T398513: Properly capitalise folders containing PHP files in MW core, it was noticed that while hierarchy exists in various folders in includes/, their namespacing doesn't match.
We should either move these classes into their correct namespacing, or move the files out of the sub folders...
See also: T353458: Make core classes follow PSR-4, T278278: Adopt PSR-4 namespaces in WMF-deployed extensions/skins