[flutter_tools] Resolve workspace root when running widget preview from member package (#190952)

When `flutter widget-preview start` is run from a member package in a
pub workspace, redirect `getRootProject()` to the workspace root
`FlutterProject`. This ensures that the widget preview scaffold is
generated at the workspace root and has dependency overrides for all
member packages in the workspace.

Fixes https://github.com/flutter/flutter/issues/190692