resources: Improve AutoEdge API and pkg breakage

I previously broke the pkg auto edges because the package list wasn't
available by the time it was called. This fixes the pkg resource so that
it gets the necessary list of packages when needed. Since this means
that a possible failure could happen, we also update the AutoEdges API
to support errors. Errors can only be generated at AutoEdge struct
creation, once the struct has been returned (right before modification
of the graph structure) there is no possibility to return any errors.

It's important to remember that the AutoEdges stuff gets called because
the Init of each resource, so make sure it doesn't depend on anything
that happens there or that gets cached as a result of Init.

This is all much nicer now and has a test too :)

lib/main.go

@@ -513,9 +513,22 @@ func (obj *Main) Run() error {
 				}
 				continue
 			}
+
+			// TODO: should we call each Res.Setup() here instead?
+
+			// add autoedges; modifies the graph only if no error
+			if err := resources.AutoEdges(oldGraph); err != nil {
+				log.Printf("Main: Error running auto edges: %v", err)
+				// unpause!
+				if !first {
+					graph.Start(first) // sync
+					converger.Start()  // after Start()
+				}
+				continue
+			}
+
 			graph.Update(oldGraph) // copy in structure of new graph
 
-			resources.AutoEdges(graph.Graph)                                      // add autoedges; modifies the graph
 			resources.AutoGroup(graph.Graph, &resources.NonReachabilityGrouper{}) // run autogroup; modifies the graph
 			// TODO: do we want to do a transitive reduction?
 			// FIXME: run a type checker that verifies all the send->recv relationships