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/tga/image_loader_tga.cpp

@@ -30,8 +30,8 @@
 
 #include "image_loader_tga.h"
 
-#include "os/os.h"
-#include "print_string.h"
+#include "core/os/os.h"
+#include "core/print_string.h"
 
 Error ImageLoaderTGA::decode_tga_rle(const uint8_t *p_compressed_buffer, size_t p_pixel_size, uint8_t *p_uncompressed_buffer, size_t p_output_size) {
 	Error error;

modules/tga/image_loader_tga.h

@@ -31,7 +31,7 @@
 #ifndef IMAGE_LOADER_TGA_H
 #define IMAGE_LOADER_TGA_H
 
-#include "io/image_loader.h"
+#include "core/io/image_loader.h"
 
 /**
 	@author SaracenOne