[augmentations] support for avoid_unused_constructor_parameters

[pq]

Example:

class A {
  A(int x); // LINT
}

augment class A {
  augment A.new(int x) { // NO LINT
    // do stuff
  }
}

[pq]

The above is now supported w/ 4e3ff1d.

The following case is interesting though (thanks @bwilkerson!)

  test_augmentedConstructor_base() async {
    newFile('$testPackageLibPath/a.dart', r'''
augment library 'test.dart';

augment class A {
  final int y;
  augment A(int x) : y = x;
}
''');

    await assertNoDiagnostics(r'''
import augment 'a.dart';

class A {
  A(int x);
}
''');
  }

The rub is that A(int x); likely shouldn't produce a lint because in the augmentation, x is referenced in the assignment to y.

To get this test to pass though, we'd need to visit the ASTs of the augmentations in AvoidUnusedConstructorParameters to ensure that parameters are unused in all of them and currently we only have elements via the augmented property of the constructor declaration.

(Note that we'll need to do similar things when evaluating if a field could be final or late or is unused or...)

/fyi @scheglov