Add `--no-emit-legacy-flutter-plugins` (which does not write `.flutter-plugins`)

[matanlurey]

Was optimistically documented in https://github.com/flutter/website/pull/11303/files.

I'd like to give our users a stable release where legacy plugins warn (see #157388, thanks @reidbaker for the suggestion), but continue to work. As part of that, I'd like to ship a hidden flag, --no-emit-legacy-flutter-plugins, that does not create a .flutter-plugins file to let end-users test their workflows/run on CI/etc.

[matanlurey]

Quick update, this will be called --legacy-pubspec-resolution instead (as a bundle of flags).

[github-actions]

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