Move graph logic into package

Graph logic moves into depgraph package and makes internal
fields inaccessible. Completes feedback from @LucasRoesler
from previous PR where the dependency graph was added for 0.9.1

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>

pkg/deployment_order.go

@@ -0,0 +1,41 @@
+package pkg
+
+import (
+	"log"
+
+	"github.com/openfaas/faasd/pkg/depgraph"
+)
+
+func buildDeploymentOrder(svcs []Service) []string {
+
+	graph := buildServiceGraph(svcs)
+
+	order := graph.Resolve()
+
+	log.Printf("Start-up order:\n")
+	for _, node := range order {
+		log.Printf("- %s\n", node)
+	}
+
+	return order
+}
+
+func buildServiceGraph(svcs []Service) *depgraph.Graph {
+	graph := depgraph.NewDepgraph()
+
+	nodeMap := map[string]*depgraph.Node{}
+	for _, s := range svcs {
+		n := &depgraph.Node{Name: s.Name}
+		nodeMap[s.Name] = n
+		graph.Add(n)
+
+	}
+
+	for _, s := range svcs {
+		for _, d := range s.DependsOn {
+			nodeMap[s.Name].Edges = append(nodeMap[s.Name].Edges, nodeMap[d])
+		}
+	}
+
+	return graph
+}