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

Change-Id: I76397394b59762f7187823efc5008c09f685e224
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/168918
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

go.mod

@@ -0,0 +1,3 @@
+module github.com/golang/protobuf
+
+require github.com/golang/protobuf/v2 v2.0.0-20190322201422-f503c300f70e

go.sum

@@ -0,0 +1,5 @@
+github.com/golang/protobuf v1.2.1-0.20190322195920-d94fb84e04b7/go.mod h1:6lQm79b+lXiMfvg/cZm0SGofjICqVBUtrP5yJMmIC1U=
+github.com/golang/protobuf/v2 v2.0.0-20190322201422-f503c300f70e h1:JQRqkjZt61BlBnTP2OpISUfb5I1LGJcqYHfFGERMmlg=
+github.com/golang/protobuf/v2 v2.0.0-20190322201422-f503c300f70e/go.mod h1:25ZALhydMFaBRgPH58a8zpFe9YXMAMjOYWtB6pNPcoo=
+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=