[ffigen] Dart API: Change `Struct.imported` and friends to a callback

[dcharkes]

final class Structs extends Declarations {
  /// Structs imported from other Dart files.
  final List<ImportedType> imported;

We should change these imported fields to callbacks:

final class Structs extends Declarations {
  /// Structs imported from other Dart files.
  final ImportedType? Function(Declaration) imported;

This will enable:

1. using Declaration.usr which means we can merge this.importedTypesByUsr = const <String, ImportedType>{} into this. ([ffigen] Dart API: Remove FfiGenerator.importedTypesByUsr #2596)
2. match imports programmatically (instead of spelling out all ImportedTypes up front)

Context:

• [ffigen] Dart API: Make more hierarchical #2559 (comment)