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.

main/tests/test_ordered_hash_map.cpp

@@ -28,10 +28,10 @@
 /* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                */
 /*************************************************************************/
 
-#include "ordered_hash_map.h"
-#include "os/os.h"
-#include "pair.h"
-#include "vector.h"
+#include "core/ordered_hash_map.h"
+#include "core/os/os.h"
+#include "core/pair.h"
+#include "core/vector.h"
 
 namespace TestOrderedHashMap {
 
@@ -168,4 +168,4 @@ MainLoop *test() {
 
 	return NULL;
 }
-} // namespace TestOrderedHashMap
+} // namespace TestOrderedHashMap