add test validating current behavior of packageOf, run CI on windows (dart-lang/package_config#137)

- validates the behavior of https://github.com/dart-lang/package_config/issues/136 today (but does not change it)
- removes build_runner deps for testing, there is no need to use it for such a small package
- fixes a bug in discovery_test.dart that was probably landed due to inability to run tests