The long-term goal is to move code out of `abc_util.{h,cc}` into either
files with better, more concrete names, or simply into the one file
where they are used.
No functional changes.
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.
