Provide utility or recipe for referencing imported library aliases

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](https://github.com/google/built_value.dart/pull/703/files#diff-e86ce28f4538fe998a497569897b4a33)

Start by reading the discussion in #109, focusing on how a DartType and its element model identify the library and required import prefix. Compare that approach with the imported-library AST parsing shown in the linked built_value pull request. Done means a reviewed utility or recipe with sample code that addresses this code-generation use case.

Written by the indexing model from the issue text.