New :class:`pluggy.ProjectSpec` hub bundles a project's
:class:`~pluggy.HookspecMarker`, :class:`~pluggy.HookimplMarker` and
:meth:`~pluggy.ProjectSpec.create_plugin_manager` under one project name.
:class:`~pluggy.HookspecMarker`, :class:`~pluggy.HookimplMarker` and
:class:`~pluggy.PluginManager` now accept either a project name string or
a ProjectSpec instance; plain strings keep working unchanged.