Refactor PixelClusteringToolBase (-> PixelRDOTool) and MergedPixelsTool

This is an MR to prepare for the implementation of pixel clustering with ACTS (see !55150 (merged)).

The current InDet classes used for clustering are MergedPixelTool & its base class, PixelClusteringToolBase. In this MR, that code is refactored to bring out of MergedPixelsTool all the RDO validation logic that will also be used by the ACTS-based version. The base class is then turned into a standalone tool that can be used by either version for this purpose (I think that's much more sound than cross-package inheritance). I also improved the readability of the existing code where I could, since it was getting out of hand.

Pinging @cvarni