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

[ajm188]

Part of #10697.

Current flags:

$ git grep -E "\bflag\.[A-Z]" -- go/internal/flag/*.go
go/internal/flag/flag.go:// Parse wraps the standard library's flag.Parse to perform some sanity checking
go/internal/flag/flag.go:func Parse(fs *flag.FlagSet) {
go/internal/flag/flag.go:       flag.CommandLine = fs
go/internal/flag/flag.go:       flag.Parse()
go/internal/flag/flag.go:       flag.Usage()
go/internal/flag/flag.go:       fs := flag.NewFlagSet("test", flag.ExitOnError)
go/internal/flag/flag.go:       return goflag.Parsed() || flag.Parsed()
go/internal/flag/flag.go:func Lookup(name string) *flag.Flag {
go/internal/flag/flag.go:       if f := flag.Lookup(name); f != nil {
go/internal/flag/flag.go:               return flag.PFlagFromGoFlag(f)
go/internal/flag/flag.go:// equivalent to flag.Args() from the standard library flag package.
go/internal/flag/flag.go:       for i, arg := range flag.Args() {
go/internal/flag/flag.go:               args = append(args, flag.Args()[doubleDashIdx+1:]...)
go/internal/flag/flag.go:// specified, this is equivalent to flag.Arg(i) from the standard library flag
go/internal/flag/usage.go:                      //      >       flag.String("I", "", "search `directory` for include files")

[ajm188]

This is just for formal tracking; in this package pflag is import-aliased to flag, and the standard library flag is import-aliased to goflag.

Because this package is doing the shim code to shuffle back and forth between the two in order to maintain compatibility during the migration, there's no cleanup to do here, but after the migration is done we will be able to delete this package entirely.