TODO: Grapher may soon have fields that need to be initialized, e.g.

Graph is a DAG where nodes are node specs (if a sequence graph) or jobs (if a request graph), and a directed edge (u, v) exists iff u is a dependency of v.

Grapher builds sequence graphs.

Node represents a node spec (if a sequence graph) or a job (if a request graph).

Resolver builds a request graph for a specific request.

ResolverFactory returns Resolvers tailored to create request graphs for a specific input request.