[CLEANUP] ReferenceIndex

Use imports, correct typos, use getters instead of globals, improve
variable names, introduce some early returns, empty() instead of
count() and explicit return statements.

Deprecate unused property $errorLog and unused function error().

Resolves: #65465
Releases: master
Change-Id: I4a7f1b5c2b74c8e5b8a7017cd05929c85f2eba07
Reviewed-on: http://review.typo3.org/37473


Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>
Reviewed-by: Andreas Fernandez <andreas.fernandez@aspedia.de>
Tested-by: Andreas Fernandez <andreas.fernandez@aspedia.de>