refactor(composer): hold managers behind *Interface traits

Composer/PartialComposer exposed its RepositoryManager, InstallationManager,
EventDispatcher, Locker, DownloadManager, AutoloadGenerator and ArchiveManager
as concrete types, but Composer's public setters (setDownloadManager() etc.)
let plugins swap in subclasses. Introduce a *Interface trait per manager and
store each as Rc<RefCell<dyn ...Interface>> so a replacement is honored.

Only Composer's slots and the sinks fed from its accessors become trait
objects; managers injected concretely at construction keep their concrete
references, matching PHP semantics. Fluent setters on the affected classes now
return () and Locker::update_hash is de-generified to a boxed FnOnce so the
traits stay object-safe.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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