Provide utility or recipe for referencing imported library aliases

[micimize]

As referencing imported symbols is a common code generation need, it would be nice to provide a specific utility or recipe (with sample code) for doing so

As noted in #109:

If you are generating code, and you have a DartType, and you want to know, "what prefix do I need to emit in order to produce a valid reference to this DartType", then using the element model to find the library is probably your best answer

It looks like built_value parses the imported library's ast