Blocked on #157462 landing first.

Similar to what was done for Swift Package Manager, e.g.

flutter config --explicit-package-resolution

Or later to opt-out:

flutter config --no-explicit-package-resolution

Advantages:

• Fits the formula of "something we are rolling out" better.
• Can be opt-out/in globally, or potentially per-project (i.e. using pubspec.yaml, which is additional custom work)

Disadvantages:

• Slightly more work, slightly more cumbersome to integration test

Steps:

• Rename and remove the top-level command-like flag
• Update the docs on https://docs.flutter.dev/release/breaking-changes

Feature flags are always positive things (adding a feature), so I'd want to call it explicit, explicit-package-resolution?