[TYPO3-core] Use "namespace" directive for relative namespaces

[Christian Müller]

Hi Bernhard,

speaking as a Flow developer we use namespacedeclarations wherever they
make sense. Which is quite a lot actually. So I would encourage the use
in TYPO3 core. It makes the files somuch more readable. And any IDE with
namespacesupport will work that out for you.

Cheers,
Christian