code: <P extends PatchingArtifact.ArtifactState, S extends PatchingArtifact.ArtifactState> PatchingArtifactStateHandler<S> getHandlerForArtifact(PatchingArtifact<P, S> artifact) {
          return handlers.get(artifact);
      }
docs: Get a state handler for a given patching artifact.
  
  @param artifact the patching artifact
  @param <P>
  @param <S>
  @return the state handler, {@code null} if there is no handler registered for the given artifact