Provide taped-package suggestion hints

If tape assist is running and users create a `@TapeClass` that has a field of an external type, i.e. Flutter's `Color`, encoding won't work without registering an `AdapterFor`. It would be amazing if instead of failing at runtime, we could see that an `AdapterForColor` is missing in the `tape.dart`, lookup the package of the type (`package:flutter`), check if a `taped_flutter` package exists on pub.dev, and suggest that to the user.

Start with the tape assist flow and the generated tape.dart handling for a missing AdapterFor external type. Trace how the external package, such as package:flutter, could be identified and how pub.dev availability would be checked. Done means users receive a taped package suggestion instead of only a runtime encoding failure.

Written by the indexing model from the issue text.