Add fetch_dependencies and fetch_dependency_graph util functions

[ravwojdyla]

It would be a useful function to have. Given an Artifact return all dependencies (recursively) with a given Artifact spec (id, optionally version/partition):

def fetch_dependencies(a: Artifact, a_dep_spec: Optional[Artifact] = None) -> Set[Artifact]:
  ...

and we recursively walk the deps from a, and compare then with the a_dep_spec, return all matches. And if a_dep_spec is None, we get a full set of dependencies. Internally this could implement say:

def fetch_dependency_graph(a: Artifact, max_depth: int = -1) -> Dict[Artifact, ...]:
 ...

Note: pseudocode above uses set/dict of Artifact, see #9.