Minor tweak for consistency
Internalize the mainSources FunctionalSourceSet into ComponentSpec.getSources()

- All LanguageSourceSets added to a component’s main source set are automatically part of the components sources
- Don’t use a rule to accomplish this: it’s part of the core component behaviour
- Once SourceSets are managed by model rules, this mechanism should be simpler

+review REVIEW-5076
Updated release notes for changes to native and jvm-lang plugins