[Validator] Allow specifying only the subspec name

lib/cocoapods/validator.rb

@@ -67,7 +67,8 @@ module Pod
       # Replace default spec with a subspec if asked for
       a_spec = spec
       if spec && @only_subspec
-        a_spec = spec.subspec_by_name(@only_subspec)
+        subspec_name = @only_subspec.start_with?(spec.root.name) ? @only_subspec : "#{spec.root.name}/#{@only_subspec}"
+        a_spec = spec.subspec_by_name(subspec_name)
         @subspec_name = a_spec.name
       end