package info: display and resolve via new source: config #1400

Part of #1294 (Phase 8 of epic #1254). Branch: new-resolver-config.

What needs to happen

Update _resolve_fromager() in src/fromager/commands/package.py:

1. Display: when pbi.source_resolver is present, show the source: config (provider name, resolves_prebuilt_wheel, min_release_age) instead of / alongside the legacy resolver_dist / download_source fields.
2. Resolve: the function currently calls overrides.find_and_invoke("get_resolver_provider", ...) directly, bypassing the new config. Switch it to sources.get_source_provider(), which already dispatches on pbi.source_resolver and falls back to the legacy path.

Tests

• package info on a source:-config package shows the source block and resolves via the configured provider
• Legacy-config packages display unchanged

Notes

Independent of the other #1294 sub-issues; can be worked on in any order.