[flutter_tools] find the command name regardless of argument position (#191155)

`flutter --no-version-check doctor -v` dumps the full command log, same
as `-vv` would. Any global option in front of the command does it, and
`help` has the same problem.

The muting check looks for the command by position:

```dart
final bool doctor =
    (args.isNotEmpty && args.first == 'doctor') ||
    (args.length == 2 && verbose && args.last == 'doctor');
```

`['--no-version-check', 'doctor', '-v']` is neither two arguments long
nor starts with `doctor`, so it falls through and nothing gets muted.

Parse the global options with a throwaway
`FlutterCommandRunner().argParser` instead and take whatever it stops
on. It has `allowTrailingOptions: false`, so the command ends up at the
head of `rest`. Going through the real parser also keeps `flutter
--local-engine host_debug doctor -v` from mistaking `host_debug` for the
command, which a plain "first argument without a dash" scan would.

Before:

    $ flutter --no-version-check doctor -v | wc -l
    1542

After:

    $ flutter --no-version-check doctor -v | wc -l
    40

`-vv` still forces the log, and `doctor -v` still prints the extra
diagnostics, since `DoctorCommand` gets `verbose` separately from the
muting.

I left the `args.length == 1 && verbose` clause in the help check alone.
It only catches a bare `flutter -v`, and widening it to "no command at
all" would start muting `flutter --version -v`.

Fixes https://github.com/flutter/flutter/issues/124793

I did went over the checklist before this PR