vulkan: add layers_extensions.h

Move everything for layers_extensions.cpp out of loader.h to
layers_extensions.h, and into vulkan::api namespace.

Remove now unused InstanceExtensionFromName and DeviceExtensionFromName.

Change-Id: I06be98986f40de35e9dffb5499bc1423e9eb3d48
diff --git a/vulkan/libvulkan/api.cpp b/vulkan/libvulkan/api.cpp
index 9c5aa3b..20f02f1 100644
--- a/vulkan/libvulkan/api.cpp
+++ b/vulkan/libvulkan/api.cpp
@@ -33,7 +33,7 @@
 #include <vulkan/vk_layer_interface.h>
 #include "api.h"
 #include "driver.h"
-#include "loader.h"
+#include "layers_extensions.h"
 
 namespace vulkan {
 namespace api {