pgraph: Clean up pgraph module to get ready for clean lib status

The graph of dependencies in golang is a DAG, and as such doesn't allow
cycles. Clean up this lib so that it eventually doesn't import our
resources module or anything else which might want to import it.

This patch makes adjacency private, and adds a generalized key store to
the graph struct.

test/shell/libmgmt-change1.go

@@ -57,7 +57,10 @@ func (obj *MyGAPI) Graph() (*pgraph.Graph, error) {
 	}
 	// FIXME: these are being specified temporarily until it's the default!
 	metaparams := resources.DefaultMetaParams
-	g := pgraph.NewGraph(obj.Name)
+	g, err := pgraph.NewGraph(obj.Name)
+	if err != nil {
+		return nil, err
+	}
 
 	n0 := &resources.NoopRes{
 		BaseRes: resources.BaseRes{