[flutter_plugin_tools] Fix pubspec-check on Windows (#4428)

The repository check always failed when run on Windows, because the
relative path generated to check the end of the URL was using local
filesystem style for separators, but URLs always use POSIX separators.

script/tool/lib/src/pubspec_check_command.dart

@@ -177,7 +177,7 @@ class PubspecCheckCommand extends PackageLoopingCommand {
       errorMessages.add('Missing "repository"');
     } else {
       final String relativePackagePath =
-          path.relative(package.path, from: packagesDir.parent.path);
+          getRelativePosixPath(package.directory, from: packagesDir.parent);
       if (!pubspec.repository!.path.endsWith(relativePackagePath)) {
         errorMessages
             .add('The "repository" link should end with the package path.');