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

[ajm188]

Part of #10697.

Current flags:

$ git grep -E "\bflag\.[A-Z]" -- go/vt/sqlparser/*.go
go/vt/sqlparser/parser.go:      if flag.Parsed() {
go/vt/sqlparser/truncate_query.go:      TruncateUILen = flag.Int("sql-max-length-ui", 512, "truncate queries in debug UIs to the given length (default 512)")
go/vt/sqlparser/truncate_query.go:      TruncateErrLen = flag.Int("sql-max-length-errors", 0, "truncate queries in error logs to the given length (default unlimited)")