Review items for 'Make branch attribute available when publishing and resolving Ivy modules' story

- Moved NamespaceId and IvyModuleDescriptor to artifacts.ivy subpackage
- Added test for NamespaceId
- Updated javadoc comment for IvyModuleDescriptor.getIvyStatus to reflect that it is immutable
+review REVIEW-5059
Review items for 'Make branch attribute available when publishing and resolving Ivy modules' story

- Changed extraInfo from a Map to an IvyExtraInfo with a map view
+review REVIEW-5059