Propagate task dependencies on API elements

[lacasseio]

When depending on a project that generates public headers through a task, the task dependencies aren't propagated to the consumer. This induces flakiness in the build as the task graph will be missing dependencies. The task dependencies are missing from the API elements.

[pedrolamarao]

Before discovering issue #994 in the issue tracker, we encountered this limitation and produced the following sample for discussion: https://github.com/pedrolamarao/gradle-native-samples/tree/master/generated-public-headers Thanks for tracking this issue. We will wait for it to be closed before proceeding with our migration. We can contribute testing.