Inspector infers correct pub root directory for widget previews

[elliette]

Fixes #8807

I'm assuming we never want to set a directory in .dart_tool as a pub root directory, but in case we do we can change the logic to specifically only exclude the widget_previews_scaffold directory.

[bkonyi]

Would build not be a better directory to look for if we have a running application?

Also, I'm not sure what the difference between the 'lib' / 'web' case and 'packages', but I assume there's a reason since that's just a refactor... :)

Overall, LGTM.

[elliette]

Would build not be a better directory to look for if we have a running application?

Also, I'm not sure what the difference between the 'lib' / 'web' case and 'packages', but I assume there's a reason since that's just a refactor... :)

Overall, LGTM.

Possibly! But I don't want to change the existing logic (which currently works) unless there's a need to