Stabilize ordering graph

In this way when we order, we always return the same solution order in
case there are weak deps.

The following is optional - it doesn't change the "correctness" of the
solver results: We add an extra edge between deps that
share common dependendencies. This makes the link more stronger and
balances the graph so it doesn't show different results for the same query, as they
could be shuffled as don't have a direct connection.

pkg/solver/decoder_test.go

@@ -388,5 +388,37 @@ var _ = Describe("Decoder", func() {
 
 			Expect(solution4.Drop(Y).AssertionHash()).To(Equal(solution4.HashFrom(Y)))
 		})
+		for index := 0; index < 300; index++ { // Just to make sure we don't have false positives
+
+			It("Always same solution", func() {
+
+				X := pkg.NewPackage("X", "", []*pkg.DefaultPackage{}, []*pkg.DefaultPackage{})
+				Y := pkg.NewPackage("Y", "", []*pkg.DefaultPackage{X}, []*pkg.DefaultPackage{})
+				Z := pkg.NewPackage("Z", "", []*pkg.DefaultPackage{X}, []*pkg.DefaultPackage{})
+				W := pkg.NewPackage("W", "", []*pkg.DefaultPackage{Z, Y}, []*pkg.DefaultPackage{})
+
+				for _, p := range []pkg.Package{X, Y, Z} {
+					_, err := dbDefinitions.CreatePackage(p)
+					Expect(err).ToNot(HaveOccurred())
+				}
+
+				for _, p := range []pkg.Package{} {
+					_, err := dbInstalled.CreatePackage(p)
+					Expect(err).ToNot(HaveOccurred())
+				}
+
+				solution, err := s.Install([]pkg.Package{W})
+				Expect(err).ToNot(HaveOccurred())
+
+				orderW, err := solution.Order(dbDefinitions, W.GetFingerPrint())
+				Expect(err).ToNot(HaveOccurred())
+				Expect(orderW[0].Package.GetName()).To(Equal("X"))
+				Expect(orderW[1].Package.GetName()).To(Equal("Y"))
+				Expect(orderW[2].Package.GetName()).To(Equal("Z"))
+				Expect(orderW[3].Package.GetName()).To(Equal("W"))
+			})
+
+		}
+
 	})
 })