Calling runcli with project set to None results in
setting the directory to the string 'None'.

Instead, only set it when the given project argument is not None.

Also update documentation in plugin for APIs that do accept Node and
not `dict`.

Calls to `isinstance` can be particularily costly. Using type() is
much faster. The only known case where the `isinstance` was useful
is for dictionnaries where we would ask for a 'Mapping' instead.

Disallowing 'Mapping' for expected_type considerably speeds up the
calls to this functions.

Also add into NEWS

This will replace the previous use of a directory structure.
Quite a lot is changed here, predominantly _artifact and _artifactcache
modules.

Part of #974

This is a follow-up to !1257 - YAML New World Order.
Now that !1257 has landed, plugin's *must* use the Plugin.node_*
family of functions to access or modify node members.