MCReconstructible: use toolhandle

This is a follow up to !2520 (merged) (and sits on top of it), which exposed an underlying problem, namely that using ToolHandle can cause spurious public tool configurable to be instantiated, even though the tool is private. As a result, there is 'squatting' on the public tool name, which can cause conflicts with other code. See the discussion [here]!2520 (comment 3384564)).

requires gaudi/Gaudi!1131 (merged)