Activate `source` field on PackageSettings/VariantInfo and add `source_resolver` PBI property

[smoparth]

Context

The new SourceResolver data model is complete in _resolver.py (PRs #1228, #1235), but the source field is still commented out on PackageSettings and VariantInfo in _models.py, and PackageBuildInfo has no source_resolver property. This is the foundation that all other wiring work depends on.

Parent epic: #1254 (Phase 1 — critical path, blocks all other phases)

Problem

The new resolver config cannot be used at runtime because the source field isn't active on the settings models and there's no way to access it through PackageBuildInfo.

Acceptance criteria

• source: SourceResolver | None field is active on both PackageSettings and VariantInfo
• SourceResolver is importable from fromager.packagesettings
• PackageBuildInfo has a source_resolver property that returns variant-level source if set, else package-level source, else None
• Loading YAML with source: config produces the correct SourceResolver instance
• pbi.source_resolver returns None when no source: is configured (legacy config unchanged)
• All existing tests pass (no legacy behavior change)

Files

• src/fromager/packagesettings/_models.py
• src/fromager/packagesettings/_pbi.py
• src/fromager/packagesettings/__init__.py

Related

• Add mutual exclusivity validator for source vs legacy fields #1207 — Mutual exclusivity validation (can be done as part of this or separately)
• Depends on: nothing
• Blocks: Phase 2, 3, 5, 6, 7, 8, 9