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/python/intern/CMakeLists.txt

@@ -305,14 +305,14 @@ endif()
 if(WITH_ALEMBIC)
   add_definitions(-DWITH_ALEMBIC)
   list(APPEND INC
-    ../../alembic
+    ../../io/alembic
   )
 endif()
 
 if(WITH_USD)
   add_definitions(-DWITH_USD)
   list(APPEND INC
-    ../../usd
+    ../../io/usd
   )
 endif()