Module validation is broken when there are both optional and required dependencies in the current context

If the current product has multiple dependencies on a module, some optional and some required, the module validation may only run once where it is optional and then will silently succeed validation (even though validation fails) when it is required.

For example, numerous modules depend on the xcode module (optional). If you create a CppApplication product and explicitly depend on the xcode module with an incorrect property set (try xcode.sdkVersion:2), no failure is reported.