Is the interface uses clause part of the API?

Dart gets close to this from the other side. Privacy per library instead of per class gets you most of it without the two sections. In Dart an identifier that starts with _ is visible everywhere in its library, part files included, and nowhere outside it. The public face of a package is usually one file under lib/ that does export with show lists, and the real code sits in lib/src, which other packages aren't supposed to import directly. It's your interface section again, spread over a file and a folder convention.

A folder convention is a polite request, not the same idea. Java had only that until a module-info.java saying exports com.example.api made every other package unreachable from other modules, public classes included.

Since the comparison is with a split the compiler enforces, like Delphi's, worth adding that this only holds when the jar actually runs on the module path. Put the same jar on the class path and it lands in the unnamed module, where the exports list does nothing. Package-private access is the cheap version and it works either way, and for an older application I can live with package-private plus a naming convention. How much code needs to change to add a module descriptor to an app that never had one?