Cleanup: move Alembic, AVI, Collada, and USD to source/blender/io

This moves the `alembic`, `avi`, `collada`, and `usd` modules into a common
`io` directory.

This also cleans up some `#include "../../{somedir}/{somefile}.h"` by
adding `../../io/{somedir}` to `CMakeLists.txt` and then just using
`#include "{somefile}.h"`.

No functional changes.

source/blender/io/avi/intern/avi_endian.h

@@ -0,0 +1,40 @@
+/*
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * The Original Code is Copyright (C) 2001-2002 by NaN Holding BV.
+ * All rights reserved.
+ */
+
+/** \file
+ * \ingroup avi
+ *
+ * This is external code.
+ */
+
+#ifndef __AVI_ENDIAN_H__
+#define __AVI_ENDIAN_H__
+
+#define AVI_RAW 0
+#define AVI_CHUNK 1
+#define AVI_LIST 2
+#define AVI_MAINH 3
+#define AVI_STREAMH 4
+#define AVI_BITMAPH 5
+#define AVI_INDEXE 6
+#define AVI_MJPEGU 7
+
+void awrite(AviMovie *movie, void *datain, int block, int size, FILE *fp, int type);
+
+#endif /* __AVI_ENDIAN_H__ */