Switch flag definitions to be on pflag instead of flag in `package go/vt/vtctl/vtctldclient/codegen`

[ajm188]

Part of #10697.

Current flags:

$ git grep -E "\bflag\.[A-Z]" -- go/vt/vtctl/vtctldclient/codegen/*.go
go/vt/vtctl/vtctldclient/codegen/main.go:       source := flag.String("source", "../../proto/vtctlservice", "source package")
go/vt/vtctl/vtctldclient/codegen/main.go:       typeName := flag.String("type", "VtctldClient", "interface type to implement")
go/vt/vtctl/vtctldclient/codegen/main.go:       implType := flag.String("impl", "gRPCVtctldClient", "type implementing the interface")
go/vt/vtctl/vtctldclient/codegen/main.go:       pkgName := flag.String("targetpkg", "grpcvtctldclient", "package name to generate code for")
go/vt/vtctl/vtctldclient/codegen/main.go:       local := flag.Bool("local", false, "generate a local, in-process client rather than a grpcclient")
go/vt/vtctl/vtctldclient/codegen/main.go:       out := flag.String("out", "", "output destination. leave empty to use stdout")
go/vt/vtctl/vtctldclient/codegen/main.go:       flag.Parse()