Behaviour for nbpr artefact resolvers.

Each resolver maps a site spec (e.g. {:github_releases, "owner/repo"}) plus build inputs to a fetch plan, then performs the actual fetch.

plan/2 is a pure function: it returns {__MODULE__, plan_map} if the resolver handles this kind of site, or nil to skip. The fetcher iterates resolvers per site and uses the first non-nil plan it gets.

get/2 performs the actual HTTP download to the destination path. It must clean up partial downloads on failure.