Fixed creation of LinksByKey in PrTrackAssociator by setting source LinkId properly

This was overlooked when MC checking was reworked. It leads to silent failure in cases where new MC tools are used to create a LinksByKey object while old code is reading it.

Now this is typically the case in a transition period where I changed the commited code and users have old code in their own places. In order to ease the transition, this commit allows the old code to run.