Change FlutterPluginsLibraryManager to use Dart plugin

[stevemessick]

In order to close #5614 we added some code that uses the package config file instead of the packages file, but it does not implement a proper cache. We were (and still are) using the code in the Dart plugin. Once the Dart plugin is updated we want to resume using that code to load the package config file. This un-cached code will only be used in the unlikely chance that we generate a project that does not have a. .packages file prior to the Dart plugin being updated.

[stevemessick]

@jwren The Dart plugin still uses .packages in one place. That's going to break when pub stops generating it.