Difference between open3d_find_package_3rdparty_library and open3d_import_3rdparty_library ?

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### My Question

It looks to me:

Both **open3d_find_package_3rdparty_library** and **open3d_import_3rdparty_library** can load libraries ...

**find_package** is looking for exisiting (system-installed) packages, while
**import** is to download 3rd-party libraries?

Anyway, can anybody help to confirm this??

Thank you ..