docs/qapi-domain: always store fully qualified name in signode

docs/qapi-domain: always store fully qualified name in signode

Currently, only the definition name is stored in the tree metadata; but
the node property is confusingly called "fullname". Rectify this by
always storing the FQN in the tree metadata.

... While we're here, re-organize the code in preparation for namespace
support to make it a bit easier to add additional components of the
FQN. With this change, there is now extremely little code left that's
taken directly from the Python domain :)

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250313044312.189276-3-jsnow@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>