[TASK] Fluid: Fix missing classes

When TYPO3 v8.0 switched to Fluid standalone, the RootNode
class was removed but the references were kept and done via
class aliases.

The Fluid standalone class should be used instead of the
class alias.

Resolves: #82335
Releases: master, 8.7
Change-Id: I50f74b71b86fcc347dc6bffbde052e6e2adbc842
Reviewed-on: https://review.typo3.org/53945


Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>