Allow hook profiles to use standard resolution with a custom download hook #1387

Summary
Support packages that provide download_source without requiring them to also provide get_resolver_provider.

Context
The current hook-sdist and hook-prebuilt implementation requires a get_resolver_provider override, while it already permits download_source to be absent and uses a standard download in that case. This prevents the reverse combination: standard resolution followed by a custom download.
The resolver proposal describes both hook profiles as using the existing hooks.

Acceptance Criteria

• hook-sdist works with a download_source override and no get_resolver_provider override, using standard source resolution.
• hook-prebuilt works with a download_source override and no get_resolver_provider override, using standard prebuilt-wheel resolution.
• Packages that provide both overrides, or only a resolver override, continue to work.
• Tests cover those combinations for both profiles, including the artifact kind returned by the download.
• Documentation states which overrides are optional and what each profile does when one is absent.