feat(package): implement PackageInterface accessor layer for Package/CompletePackage/RootPackage

Fill the trait-impl todo!() across the package accessor layer so loaded
packages are actually usable (the ArrayLoader::load path depends on it):

- Package: implement BasePackage/PackageInterface from struct fields and
  existing inherent methods; Display via get_unique_name.
- CompletePackage: delegate PackageInterface to inner Package.
- RootPackage: delegate CompletePackageInterface/PackageInterface to inner
  CompletePackage; RootPackageInterface link setters delegate to Package.

Correct three unfaithful trait signatures found during implementation:

- get_target_dir returns Option<String> (PHP computes a normalized value;
  a borrow cannot represent it, and AliasPackage could not implement &str
  across its aliasOf handle).
- RootPackageInterface link setters take IndexMap<String, Link>, matching
  Package and the real ArrayLoader caller (PHP RootPackage inherits
  Package::setRequires; the Link[] docblock was imprecise).
- get_full_pretty_version takes a DisplayMode enum instead of a raw i64;
  the match is now exhaustive, so it returns String without an error path.

Move mirror conversion to the boundaries: PackageInterface mirror methods
use Vec<Mirror> (the typed form, matching the inherent methods), with
array<->Mirror conversion done by the producer (ComposerRepository) and
consumer (ArrayDumper).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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