Switch flag definitions to be on pflag instead of flag in `package go/cmd/vtctldclient`

Part of https://github.com/vitessio/vitess/issues/10697.

Current flags:

```
$ git grep -E "\bflag\.[A-Z]" -- go/cmd/vtctldclient/*.go
go/cmd/vtctldclient/main.go:    command.Root.PersistentFlags().AddGoFlagSet(flag.CommandLine)
go/cmd/vtctldclient/main.go:    // hack to get rid of an "ERROR: logging before flag.Parse"
go/cmd/vtctldclient/main.go:    flag.Parse()
```