REVIEW-4074 rename objcpp plugin to objective-cpp plugin and change convention directories accordingly
REVIEW-4074: replace full qualified class references
Some cleanup on objective-cpp plugin:

- REVIEW-4074 add unit test coverage on ObjectiveCppNativeBinariesPlugin
- Generate sourcesets only for non empty sourcesets
- let DefaultObjectiveCppSourceSet implement ObjectiveCppSourceSet instead of DefaultObjectiveCSourceSet