• Adds discovery API to find both a configuration and its location:
  findPackageConfigAndFile and findPackageConfigAndUri.
• Removes support for the .packages file.
  The Dart SDK no longer supports that file, and no new .packages files
  will be generated.
  Since the SDK requirement for this package is above 3.0.0,
  no supporting SDK can use or generate .packages.
• Simplifies API that no longer needs to support two separate files.
  • Renamed readAnyConfigFile to readConfigFile, and removed
    the preferNewest parameter.
  • Same for readAnyConfigFileUri which becomes readConfigFileUri.
  • Old functions still exists as deprecated, forwarding to the new
    functions without the preferNewest argument.
    Makes PackageConfig, Package and LanguageVersion @sealed classes,
    in preparation for making them final in a future update.
• Adds PackageConfig.minVersion to complement .maxVersion.
  Currently both are 2.