The goal of the `AbstractHierarchyIterator` class (and supporting
classes) was to use it in different exporters. It shouldn't be part of
the USD module + namespace any more, now that it will also be used in
the upcoming Alembic exporter rewrite.
The source files are moved into `io/common`, which is compiled & linked
into a new library `bf_io_common`. The unittests are still inside the
`tests/gtests/usd` directory. They should be moved to a separate test
module too, but that will be delayed until after T73268 has been
resolved.
Reviewed By: mont29
Differential Revision: https://developer.blender.org/D7669
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.
