DispatchLoader-related classes should probably not be in the `detail` namespace

[maxime-modulopi]

DispatchLoader-related classes have multiple public usages, including use via VULKAN_HPP_DEFAULT_DISPATCHER or as last argument of every Vulkan function call when VULKAN_HPP_NO_DEFAULT_DISPATCHER is defined, in which case the user is actually required to manage its own instance(s) of these classes.