Rename internal/{checker,service,provider,notification} to
internal/{dnschecker,serviceanalyzer,providerregistry,notifier} so
imports no longer collide with the same-named usecase subpackages and
no longer require import aliases to disambiguate.
Several service analyzers were silently discarding errors returned by
a.UseRR(), unlike txt.go and others that properly checked them. This
could mask issues like double-claimed records during zone analysis.
Analogous to internal/provider, extract the service registry (Svc,
RegisterService, FindService, ListServices, OrderedServices, FindSubService,
RegisterSubServices) and the zone analyzer (ServiceAnalyzer, Analyzer,
AnalyzeZone) from services/ into a new internal/service package.
- Moved usecases into internal/usecase
- Renamed internal/utils to internal/helpers for better naming semantics
- Clarified separation between domain model, usecases, adapters, and web/API
