Class modifiers in multi-library packages

Class modifiers are a very intriguing feature of Dart 3, but at present they're difficult to use in a project that otherwise follows general Dart best practices of separating concerns into different libraries that only expose the necessary APIs at library boundaries. If you want to get the benefits of exhaustiveness checking for a given class hierarchy, for example, you either have to include the entire hierarchy in one massive library, or use `part of` which obfuscates loads and references. Both of these solutions prevent effective cross-library encapsulation.

This is, admittedly, a difficult problem given that the Dart language doesn't have any notion of multi-file groups like "packages". That said, I'm hoping there's a middle ground that could be reached to allow these modifiers to be useful without pulling everything into the same file. As a straw example, you could treat libraries that are `import`ed by a class's library as "in the same library" for class modifier purposes. This would ensure that you can still determine the full scope of a `sealed` class by resolving its library.

Start by reading the Dart 3 class modifiers and library-boundary rules, focusing on sealed hierarchies, imports, and part-of libraries. Compare the current exhaustiveness and visibility behavior with the proposed multi-library model; done means a concrete, specified approach that preserves encapsulation and defines how the full sealed hierarchy is resolved.

Written by the indexing model from the issue text.