Avoid duplicate files from being returned GetManifestFilenames

[doughertym]

The current implementation of the AbstractManifestFinder.GetManifestFilenames can return the same file multiple times. The primary cause of this seems to be related to multiple IManifestFinder instance of the same type being registered more than once. Therefore, the ManifestFinderRegistry should only register an AbstractManifestFinder if it has not already been registered.