REVIEW-2177: remove unnecessary project dependency on wrapper project in plugins
Some refactorings for placeholder actions in TaskContainerInternal.

- REVIEW-2177: - Use Runnable instead of Action<Project>
               - Add more unit code coverage for placeholder actions in DefaultTaskContainer

- REVIEW-2178: - don't call of DefaultTaskContainer#findByName twice for existing tasks.
REVIEW-2184 introduce WrapperTestFixture to check generated wrapper files.