MultiHttpClient is in includes/libs, but uses MediaWiki code including MediaWikiServices, wfParseUrl, wfAssembleUrl. Currently things in libs, including RESTBagOStuff, SwiftFileBackend and VirtualRESTServiceClient all depend on it.
Some previous discussion at https://gerrit.wikimedia.org/r/c/mediawiki/core/+/736087 and https://gerrit.wikimedia.org/r/c/mediawiki/core/+/533983