all: Become a Go module (fixes #5148) (#5384)

* go mod init; rm -rf vendor

* tweak proto files and generation

* go mod vendor

* clean up build.go

* protobuf literals in tests

* downgrade gogo/protobuf

vendor/github.com/gogo/protobuf/protoc-gen-gogo/descriptor/descriptor_test.go

@@ -1,31 +0,0 @@
-package descriptor_test
-
-import (
-	"fmt"
-	"testing"
-
-	tpb "github.com/gogo/protobuf/proto/testdata"
-	"github.com/gogo/protobuf/protoc-gen-gogo/descriptor"
-)
-
-func TestMessage(t *testing.T) {
-	var msg *descriptor.DescriptorProto
-	fd, md := descriptor.ForMessage(msg)
-	if pkg, want := fd.GetPackage(), "google.protobuf"; pkg != want {
-		t.Errorf("descriptor.ForMessage(%T).GetPackage() = %q; want %q", msg, pkg, want)
-	}
-	if name, want := md.GetName(), "DescriptorProto"; name != want {
-		t.Fatalf("descriptor.ForMessage(%T).GetName() = %q; want %q", msg, name, want)
-	}
-}
-
-func Example_Options() {
-	var msg *tpb.MyMessageSet
-	_, md := descriptor.ForMessage(msg)
-	if md.GetOptions().GetMessageSetWireFormat() {
-		fmt.Printf("%v uses option message_set_wire_format.\n", md.GetName())
-	}
-
-	// Output:
-	// MyMessageSet uses option message_set_wire_format.
-}