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.

drivers/coremidi/core_midi.cpp

@@ -31,7 +31,8 @@
 #ifdef COREMIDI_ENABLED
 
 #include "core_midi.h"
-#include "print_string.h"
+
+#include "core/print_string.h"
 
 #include <CoreAudio/HostTime.h>
 #include <CoreServices/CoreServices.h>

drivers/coremidi/core_midi.h

@@ -33,12 +33,11 @@
 #ifndef CORE_MIDI_H
 #define CORE_MIDI_H
 
-#include <stdio.h>
+#include "core/os/midi_driver.h"
+#include "core/vector.h"
 
 #include <CoreMIDI/CoreMIDI.h>
-
-#include "core/vector.h"
-#include "os/midi_driver.h"
+#include <stdio.h>
 
 class MIDIDriverCoreMidi : public MIDIDriver {