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/depgraph/depgraph_test.go

@@ -0,0 +1,41 @@
+package depgraph
+
+import "testing"
+
+func Test_RemoveMedial(t *testing.T) {
+	g := Graph{nodes: []*Node{}}
+	a := &Node{Name: "A"}
+	b := &Node{Name: "B"}
+	c := &Node{Name: "C"}
+
+	g.nodes = append(g.nodes, a)
+	g.nodes = append(g.nodes, b)
+	g.nodes = append(g.nodes, c)
+
+	g.Remove(b)
+
+	for _, n := range g.nodes {
+		if n.Name == b.Name {
+			t.Fatalf("Found deleted node: %s", n.Name)
+		}
+	}
+}
+
+func Test_RemoveFinal(t *testing.T) {
+	g := Graph{nodes: []*Node{}}
+	a := &Node{Name: "A"}
+	b := &Node{Name: "B"}
+	c := &Node{Name: "C"}
+
+	g.nodes = append(g.nodes, a)
+	g.nodes = append(g.nodes, b)
+	g.nodes = append(g.nodes, c)
+
+	g.Remove(c)
+
+	for _, n := range g.nodes {
+		if n.Name == c.Name {
+			t.Fatalf("Found deleted node: %s", c.Name)
+		}
+	}
+}