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 :)

resources/resources.go

@@ -157,7 +157,7 @@ type Res interface {
 	UIDs() []ResUID // most resources only return one
 	Watch() error   // send on channel to signal process() events
 	CheckApply(apply bool) (checkOK bool, err error)
-	AutoEdges() AutoEdge
+	AutoEdges() (AutoEdge, error)
 	Compare(Res) bool
 	CollectPattern(string) // XXX: temporary until Res collection is more advanced
 	//UnmarshalYAML(unmarshal func(interface{}) error) error // optional
@@ -463,6 +463,13 @@ func (obj *BaseRes) SetGroup(g []Res) {
 	obj.grouped = g
 }
 
+// AutoEdges returns the AutoEdge interface. By default, none are created. This
+// should be implemented by the specific resource to be used. This base method
+// does not need to be called by the resource specific implementing method.
+func (obj *BaseRes) AutoEdges() (AutoEdge, error) {
+	return nil, nil
+}
+
 // Compare is the base compare method, which also handles the metaparams cmp.
 func (obj *BaseRes) Compare(res Res) bool {
 	// TODO: should the AutoEdge values be compared?