refactor(registry): route Packagist queries through RepositorySet

Replace direct packagist::fetch_package_versions calls in
resolver::resolve (seed + transitive loops) and lockfile::generate_lock_file
with repo_set.load_packages calls. PackagistRepository now propagates
errors instead of swallowing them, so the seed loop's strictness and the
transitive loop's local-leniency are both preserved exactly.

VCS and inline-package repositories are still preloaded directly into
the pool builder for now, with their names tracked in skip lists so we
don't double-load them through the trait. Migrating them through
RepositorySet is a follow-up - vcs_to_pool_inputs and
packagist_to_pool_inputs differ in dev-branch handling that needs to be
unified first.

All 136 enabled installer fixtures + 114 mozart-registry tests + 541
mozart lib tests remain green; clippy clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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