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. Notable exceptions are klog and gogo/protobuf.

vendor/google.golang.org/grpc/reflection/README.md

@@ -0,0 +1,18 @@
+# Reflection
+
+Package reflection implements server reflection service.
+
+The service implemented is defined in: https://github.com/grpc/grpc/blob/master/src/proto/grpc/reflection/v1alpha/reflection.proto.
+
+To register server reflection on a gRPC server:
+```go
+import "google.golang.org/grpc/reflection"
+
+s := grpc.NewServer()
+pb.RegisterYourOwnServer(s, &server{})
+
+// Register reflection service on gRPC server.
+reflection.Register(s)
+
+s.Serve(lis)
+```