Declare package:flutter_gen to be deprecated - #11343

…157879)

Work towards #157819. **No behavior changes as a result of this PR**.

Based on a proof of concept by @jonahwilliams (#157818).

The existence of this flag (which for the time being, defaults to `true`) implies the following:

1. The (legacy, deprecated) `.flutter-plugins` file is not generated:
    https://docs.flutter.dev/release/breaking-changes/flutter-plugins-configuration
    
2. The (legacy, deprecated) `package:flutter_gen` is not synthetically generated:
    flutter/website#11343
    (awaiting website approvers, but owners approve this change)

This change creates `useImplicitPubspecResolution` and plumbs it through as a required variable, parsing it from a `FlutterCommand.globalResults` where able. In tests, I've defaulted the value to `true` 100% of the time - except for places where the value itself is acted on directly, in which case there are true and false test-cases (e.g. localization and i10n based classes and functions).

I'm not extremely happy this needed to change 50+ files, but is sort of a result of how inter-connected many of the elements of the tools are. I believe keeping this as an explicit (flagged) argument will be our best way to ensure the default behavior changes consistently and that tests are running as expected.