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 :)

docs/resource-guide.md

@@ -379,7 +379,7 @@ if another resource can match a dependency to this one.
 
 ### AutoEdges
 ```golang
-AutoEdges() AutoEdge
+AutoEdges() (AutoEdge, error)
 ```
 
 This returns a struct that implements the `AutoEdge` interface. This struct