apt: VersionQuerier, Holder, Cleaner, FileOwner, RepoManager, KeyManager, NameNormalizer
dpkg: FileOwner, NameNormalizer
All new interfaces follow the existing pattern of exported methods on
struct delegating to unexported platform-specific functions. Mutating
operations use Lock/Unlock; read-only operations do not.
Platform stubs in _other.go files return ErrUnsupportedPlatform.
Compile-time interface checks added for all implemented interfaces.
