replace "dep" with "go mod"

A simple "go mod init && go mod tidy && go mod vendor" was enough and
mostly seems to have picked the same revisions of the dependencies as before.

go.mod

@@ -0,0 +1,27 @@
+module github.com/kubernetes-csi/external-snapshotter
+
+go 1.12
+
+require (
+	github.com/container-storage-interface/spec v1.1.0
+	github.com/golang/groupcache v0.0.0-20190129154638-5b532d6fd5ef // indirect
+	github.com/golang/mock v1.2.0
+	github.com/golang/protobuf v1.3.1
+	github.com/google/go-cmp v0.3.1 // indirect
+	github.com/googleapis/gnostic v0.2.0 // indirect
+	github.com/hashicorp/golang-lru v0.5.1 // indirect
+	github.com/imdario/mergo v0.3.7 // indirect
+	github.com/kubernetes-csi/csi-lib-utils v0.6.1
+	github.com/kubernetes-csi/csi-test v2.0.0+incompatible
+	golang.org/x/time v0.0.0-20190308202827-9d24e82272b4 // indirect
+	google.golang.org/genproto v0.0.0-20190401181712-f467c93bbac2 // indirect
+	google.golang.org/grpc v1.19.1
+	gopkg.in/inf.v0 v0.9.1 // indirect
+	k8s.io/api v0.0.0-20190809220925-3ab596449d6f
+	k8s.io/apiextensions-apiserver v0.0.0-20190315093550-53c4693659ed
+	k8s.io/apimachinery v0.0.0-20190809020650-423f5d784010
+	k8s.io/client-go v0.0.0-20190809221226-62f300f03a5f
+	k8s.io/klog v0.3.1
+	k8s.io/kubernetes v1.14.0
+	k8s.io/utils v0.0.0-20190809000727-6c36bc71fc4a // indirect
+)