Make core/ includes absolute, remove subfolders from include path

This allows more consistency in the manner we include core headers,
where previously there would be a mix of absolute, relative and
include path-dependent includes.

modules/etc/image_etc.cpp

@@ -31,10 +31,10 @@
 #include "image_etc.h"
 #include "Etc.h"
 #include "EtcFilter.h"
-#include "image.h"
-#include "os/copymem.h"
-#include "os/os.h"
-#include "print_string.h"
+#include "core/image.h"
+#include "core/os/copymem.h"
+#include "core/os/os.h"
+#include "core/print_string.h"
 
 static Image::Format _get_etc2_mode(Image::DetectChannels format) {
 	switch (format) {

modules/etc/texture_loader_pkm.cpp

@@ -30,7 +30,7 @@
 
 #include "texture_loader_pkm.h"
 
-#include "os/file_access.h"
+#include "core/os/file_access.h"
 #include <string.h>
 
 struct ETC1Header {

modules/etc/texture_loader_pkm.h

@@ -31,7 +31,7 @@
 #ifndef TEXTURE_LOADER_PKM_H
 #define TEXTURE_LOADER_PKM_H
 
-#include "io/resource_loader.h"
+#include "core/io/resource_loader.h"
 #include "scene/resources/texture.h"
 
 class ResourceFormatPKM : public ResourceFormatLoader {