layers: simplify v0 function implementations

Since each of the layer library contains only a layer, we can simplify v0
functions by making them wrappers to the contained layer's corresponding
commands.

While we have to make changes to each layer individually, all the changes
should be the same in spirit.  The exception is vk-layer-generate.py where
the missing vkEnumerateDeviceExtensionProperties is also added.
diff --git a/layers/parameter_validation.cpp b/layers/parameter_validation.cpp
index e584d46..c70142b 100644
--- a/layers/parameter_validation.cpp
+++ b/layers/parameter_validation.cpp
@@ -4453,6 +4453,24 @@
     }
 }
 
+VKAPI_ATTR VkResult VKAPI_CALL
+EnumerateInstanceLayerProperties(uint32_t *pCount, VkLayerProperties *pProperties) {
+    return util_GetLayerProperties(1, &global_layer, pCount, pProperties);
+}
+
+VKAPI_ATTR VkResult VKAPI_CALL
+EnumerateDeviceLayerProperties(VkPhysicalDevice physicalDevice, uint32_t *pCount, VkLayerProperties *pProperties) {
+    return util_GetLayerProperties(1, &global_layer, pCount, pProperties);
+}
+
+VKAPI_ATTR VkResult VKAPI_CALL
+EnumerateInstanceExtensionProperties(const char *pLayerName, uint32_t *pCount, VkExtensionProperties *pProperties) {
+    if (pLayerName && !strcmp(pLayerName, global_layer.layerName))
+        return util_GetExtensionProperties(1, instance_extensions, pCount, pProperties);
+
+    return VK_ERROR_LAYER_NOT_PRESENT;
+}
+
 VKAPI_ATTR VkResult VKAPI_CALL EnumerateDeviceExtensionProperties(VkPhysicalDevice physicalDevice,
                                                                   const char *pLayerName, uint32_t *pCount,
                                                                   VkExtensionProperties *pProperties) {
@@ -4528,14 +4546,12 @@
         { "vkGetPhysicalDeviceSparseImageFormatProperties", reinterpret_cast<PFN_vkVoidFunction>(GetPhysicalDeviceSparseImageFormatProperties) },
         { "vkGetPhysicalDeviceQueueFamilyProperties", reinterpret_cast<PFN_vkVoidFunction>(GetPhysicalDeviceQueueFamilyProperties) },
         { "vkGetPhysicalDeviceMemoryProperties", reinterpret_cast<PFN_vkVoidFunction>(GetPhysicalDeviceMemoryProperties) },
+        { "vkEnumerateInstanceLayerProperties", reinterpret_cast<PFN_vkVoidFunction>(EnumerateInstanceLayerProperties) },
+        { "vkEnumerateDeviceLayerProperties", reinterpret_cast<PFN_vkVoidFunction>(EnumerateDeviceLayerProperties) },
+        { "vkEnumerateInstanceExtensionProperties", reinterpret_cast<PFN_vkVoidFunction>(EnumerateInstanceExtensionProperties) },
         { "vkEnumerateDeviceExtensionProperties", reinterpret_cast<PFN_vkVoidFunction>(EnumerateDeviceExtensionProperties) },
     };
 
-    // we should never be queried for these commands
-    assert(strcmp(name, "vkEnumerateInstanceLayerProperties") &&
-           strcmp(name, "vkEnumerateInstanceExtensionProperties") &&
-           strcmp(name, "vkEnumerateDeviceLayerProperties"));
-
     for (size_t i = 0; i < ARRAY_SIZE(core_instance_commands); i++) {
         if (!strcmp(core_instance_commands[i].name, name))
             return core_instance_commands[i].proc;
@@ -4698,25 +4714,26 @@
 
 VK_LAYER_EXPORT VKAPI_ATTR VkResult VKAPI_CALL
 vkEnumerateInstanceExtensionProperties(const char *pLayerName, uint32_t *pCount, VkExtensionProperties *pProperties) {
-    return util_GetExtensionProperties(1, parameter_validation::instance_extensions, pCount, pProperties);
+    return parameter_validation::EnumerateInstanceExtensionProperties(pLayerName, pCount, pProperties);
 }
 
 VK_LAYER_EXPORT VKAPI_ATTR VkResult VKAPI_CALL
 vkEnumerateInstanceLayerProperties(uint32_t *pCount, VkLayerProperties *pProperties) {
-    return util_GetLayerProperties(1, &parameter_validation::global_layer, pCount, pProperties);
+    return parameter_validation::EnumerateInstanceLayerProperties(pCount, pProperties);
 }
 
 VK_LAYER_EXPORT VKAPI_ATTR VkResult VKAPI_CALL
 vkEnumerateDeviceLayerProperties(VkPhysicalDevice physicalDevice, uint32_t *pCount, VkLayerProperties *pProperties) {
-
-    /* parameter_validation's physical device layers are the same as global */
-    return util_GetLayerProperties(1, &parameter_validation::global_layer, pCount, pProperties);
+    // the layer command handles VK_NULL_HANDLE just fine internally
+    assert(physicalDevice == VK_NULL_HANDLE);
+    return parameter_validation::EnumerateDeviceLayerProperties(VK_NULL_HANDLE, pCount, pProperties);
 }
 
 VK_LAYER_EXPORT VKAPI_ATTR VkResult VKAPI_CALL vkEnumerateDeviceExtensionProperties(VkPhysicalDevice physicalDevice,
                                                                                     const char *pLayerName, uint32_t *pCount,
                                                                                     VkExtensionProperties *pProperties) {
-    // the layer command handles VK_NULL_HANDLE just fine
+    // the layer command handles VK_NULL_HANDLE just fine internally
+    assert(physicalDevice == VK_NULL_HANDLE);
     return parameter_validation::EnumerateDeviceExtensionProperties(VK_NULL_HANDLE, pLayerName, pCount, pProperties);
 }
 
@@ -4725,14 +4742,5 @@
 }
 
 VK_LAYER_EXPORT VKAPI_ATTR PFN_vkVoidFunction VKAPI_CALL vkGetInstanceProcAddr(VkInstance instance, const char *funcName) {
-    if (!strcmp(funcName, "vkEnumerateInstanceLayerProperties"))
-        return reinterpret_cast<PFN_vkVoidFunction>(vkEnumerateInstanceLayerProperties);
-    if (!strcmp(funcName, "vkEnumerateDeviceLayerProperties"))
-        return reinterpret_cast<PFN_vkVoidFunction>(vkEnumerateDeviceLayerProperties);
-    if (!strcmp(funcName, "vkEnumerateInstanceExtensionProperties"))
-        return reinterpret_cast<PFN_vkVoidFunction>(vkEnumerateInstanceExtensionProperties);
-    if (!strcmp(funcName, "vkGetInstanceProcAddr"))
-        return reinterpret_cast<PFN_vkVoidFunction>(vkGetInstanceProcAddr);
-
     return parameter_validation::GetInstanceProcAddr(instance, funcName);
 }