[Widget Previews] Widget inspector hides widgets from user's local project in implementation widgets #8807

Steps to repro:

1. git clone https://github.com/bkonyi/widget_preview_study
2. cd widget_preview_study/widget_preview
3. flutter pub get
4. cd ../veggieseasons
5. flutter pub get
6. open widget_preview_study/veggieseasons in your IDE
7. add code below to veggie_card.dart
8. run dart run ../widget_preview/bin/find_previews.dart in IDE terminal
9. look for VM service URI and copy it
10. launch devtools from IDE, then paste in VM service URI
11. open the inspector

Expected: Widgets defined in veggie_card.dart should be considered part of the local project, and shown in the inspector tree when the implementation widgets are hidden
Actual: Widgets defined in veggie_card.dart are considered implementation widgets, and hidden in the inspector tree

Note: This is because we detect the pub root directory to be the preview scaffold, therefore anything outside of the preview scaffold is ignored:

@bkonyi for widget previews, will the preview scaffold always match the path /flutter/app/directory/.dart_tool/preview_scaffold? If so, we could check if the pub root ends with .dart_tool/preview_scaffold and instead set it to be the parent directory of .dart_tool.

import 'package:veggieseasons/data/local_veggie_provider.dart';
import 'package:widget_preview/widget_preview.dart';

@Preview()
List<WidgetPreview> veggieCardPreview() {
  return [
    WidgetPreview(
      name: 'Veggie Card',
      child: VeggieCard(LocalVeggieProvider.veggies.first, false, true),
    ),
  ];
}