Resolver#identify should return null whenever a module can't be identified

[dgeb]

This will allow for better co-existence with other resolvers, like the "unified resolver" being developed for Ember. Otherwise, we'll see performance problems related to catching assertion errors whenever identify fails.

[pittst3r]

@dgeb I'm curious if it would be appropriate to use Option? I don't have all the context for interop so this could be a bad idea, but just a thought.