Consider analysis_options excludes when deciding whether to use Dart or Flutter SDK #4028
Description
Similar to #2964, but using the existing analysis_options.yaml exclusions.
Currently if you open a project like dart-lang/linter it'll pick the Flutter SDK because of some nested integration test projects that are already excluded from analysis.
At the same time, if we bring in a YAML parser we can tidy up some string checks on pubspecs (like checking if the Flutter SDK is referenced).