GH-967 make libraries handle their own path prefix

Makes it possible to mix libraries managed by FTB and MultiMC
Backport from unstable

logic/minecraft/OneSixLibrary.cpp

@@ -29,6 +29,7 @@ OneSixLibrary::OneSixLibrary(RawLibraryPtr base)
 	m_native_classifiers = base->m_native_classifiers;
 	m_rules = base->m_rules;
 	dependType = base->dependType;
+	m_storagePrefix = base->m_storagePrefix;
 	// these only make sense for raw libraries. OneSix
 	/*
 	insertType = base->insertType;