Add `source_resolver` dispatch to resolution pipeline

[smoparth]

Context

After the source field is activated on models and pbi.source_resolver property is available (Phase 1), the resolution pipeline needs to dispatch to the new system when a package has source: config.

Parent epic: #1254 (Phase 2)

What needs to happen

Add a dispatch check in sources.resolve_source():

def resolve_source(*, ctx, req, sdist_server_url, req_type=None):
    pbi = ctx.package_build_info(req)

    if pbi.source_resolver is not None:
        # New path: source_resolver handles provider creation
        provider = pbi.source_resolver.resolver_provider(ctx, req, req_type)
        max_age_cutoff = resolver._compute_max_age_cutoff(ctx)
        results = resolver.find_all_matching_from_provider(
            provider, req, max_age_cutoff=max_age_cutoff
        )
        url, version = results[0]
        return str(url), version
    else:
        # Legacy path: unchanged
        # ... existing code using get_source_provider() ...

Similarly update sources.get_source_provider() if it has callers beyond resolve_source().

Key points

• The new resolver_provider() already handles cooldown internally (passes _cooldown to provider constructor) — no need to call resolve_package_cooldown() again on the new path
• Override hooks (get_resolver_provider) are NOT called on the new path — packages using hooks should use provider: hook-sdist or provider: hook-prebuilt in their config
• The legacy path remains completely unchanged

Files

• src/fromager/sources.py — resolve_source(), get_source_provider()

Tests

• Package with source: {provider: pypi-sdist} → new resolution path used
• Package without source: → legacy resolution path used (existing behavior)
• Package with source: {provider: hook-sdist} → hook resolver used
• Cooldown is applied correctly on new path (not double-applied)

Depends on

• Phase 1: Activate source field + PBI property

Related

• Phase 7 (New source config cooldown path has gaps compared to legacy resolve_package_cooldown() #1252): Cooldown integration builds on this
• Phase 8: Command integration depends on this