In rB7c5a44c71f13 I changed the way transform matrices are loaded from
Alembic. Instead of having the Alembic importer convert matrices from
local (in the Alembic file) to World (to pass to the constraint handling
the animation of transforms), I set the constraint space to
`CONSTRAINT_SPACE_LOCAL`.
This worked thanks to rB7728bfd4c45c. However, that commit was reverted,
which meant that for parentless objects `CONSTRAINT_SPACE_LOCAL` no
longer means "local space".
The situation is resolved by setting the constraint to world space
again, and computing the world matrix in the Alembic importer.
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.
