all: re-add go.mod and go.sum files

Change-Id: I0eed4846a597daf112016bce1dcc198aa7c3a5f1
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/168899
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

cmd/protoc-gen-go-grpc/testdata/go.mod

@@ -0,0 +1,9 @@
+module github.com/golang/protobuf/v2/cmd/protoc-gen-go-grpc/testdata
+
+require (
+	github.com/golang/protobuf v1.2.1-0.20190322195920-d94fb84e04b7
+	github.com/golang/protobuf/v2 v2.0.0
+	google.golang.org/grpc v1.19.0
+)
+
+replace github.com/golang/protobuf/v2 => ../../..

cmd/protoc-gen-go/testdata/go.mod

@@ -0,0 +1,8 @@
+module github.com/golang/protobuf/v2/cmd/protoc-gen-go/testdata
+
+require (
+	github.com/golang/protobuf v1.2.1-0.20190322195920-d94fb84e04b7
+	github.com/golang/protobuf/v2 v2.0.0
+)
+
+replace github.com/golang/protobuf/v2 => ../../..

go.mod

@@ -0,0 +1,6 @@
+module github.com/golang/protobuf/v2
+
+require (
+	github.com/golang/protobuf v1.2.1-0.20190322195920-d94fb84e04b7
+	github.com/google/go-cmp v0.2.1-0.20190312032427-6f77996f0c42
+)

go.sum

@@ -0,0 +1,4 @@
+github.com/golang/protobuf v1.2.1-0.20190322195920-d94fb84e04b7 h1:u9KfmPcHwYTSPJyTFv1ZXkTU96h4cdgvTFOS6YBai60=
+github.com/golang/protobuf v1.2.1-0.20190322195920-d94fb84e04b7/go.mod h1:6lQm79b+lXiMfvg/cZm0SGofjICqVBUtrP5yJMmIC1U=
+github.com/google/go-cmp v0.2.1-0.20190312032427-6f77996f0c42 h1:q3pnF5JFBNRz8sRD+IRj7Y6DMyYGTNqnZ9axTbSfoNI=
+github.com/google/go-cmp v0.2.1-0.20190312032427-6f77996f0c42/go.mod h1:8QqcDgzrUqlUb/G2PQTWiueGozuR1884gddMywk6iLU=