Ruby: Restrict use-use flow

[hvitved]

Same as e.g. ec5d8ab.

[aschackmull]

The context here is the public predicate localSsaFlowStep(Ssa::Definition def, Node nodeFrom, Node nodeTo), which now no longer includes the regular use-use steps. Currently this is fine, as all its references are in this file and have suitable additions of LocalFlow::localSsaFlowStepUseUse(_, nodeFrom, nodeTo), but this makes the public exposure of localSsaFlowStep (through an internal qll, but still) somewhat shaky. Also, the def column of the predicate is unused. I suggest making localSsaFlowStep private and removing its def column.

[hvitved]

I moved localFlowStepCommon into the LocalFlow module (where it actually belongs), and could then make the predicate private.

[aschackmull]

You just pushed the bubble. Granted, the useless column is now gone, but the semantic issue has just moved to localFlowStepCommon, which is now being publicly exposed (and doesn't include the use-use step). Could you make the containing LocalFlow module private?

[hvitved]

Could you make the containing LocalFlow module private?

That module is referenced in TypeTrackerSpecific.qll, so doesn't work. Also, I don't think it is really that problematic, since DataFlowPrivate is very much an internal library.