refactor custom function target to take more arguments

[dmrickey]

refactor custom target's getSourcesFor to use _getSourcesFor instead so it's not duplicating logic. Also pass the the "this item" to the function to verify things against "this item" (i.e. if this is equipped or something like that which can't be obtained other ways).

Also potentially change signature (would be breaking) so it accepts "item" and/or "action" and "actionUse" individually which is easier for someone to consume to write their own function.