Use dictionary to track PackageRelationships instead of searching a list (#35978)

Use dictionary to track PackageRelationships instead of searching a list (#35978)

This change adds an OrderedDictionary<TKey,TValue> type to System.IO.Packaging to keep a list of relationships that have been added, while ensuring that the ids are unique with decent performance characteristics. The current implementation searches a list which gets really slow with large numbers of relationships. A Dictionary by itself does not work for this as the order of addition is important. Thus, this combines a LinkedList to track the order, while a Dictionary is used to for quick look up of existing items.