fix Open3DConfig with system libraries

[daizhirui]

Type

• Bug fix (non-breaking change which fixes an issue): Fixes #
• New feature (non-breaking change which adds functionality). Resolves #
• Breaking change (fix or feature that would cause existing functionality to not work as expected) Resolves #

Motivation and Context

Checklist:

• I have run python util/check_style.py --apply to apply Open3D code style
  to my code.
• This PR changes Open3D behavior or adds new functionality.
  • Both C++ (Doxygen) and Python (Sphinx / Google style) documentation is
    updated accordingly.
  • I have added or updated C++ and / or Python unit tests OR included test
    results (e.g. screenshots or numbers) here.
• I will follow up and update the code if CI fails.
• For fork PRs, I have selected Allow edits from maintainers.

Description

When Open3D is compiled with 3rd party libraries installed in system path, e.g. fmt in /usr, the generated Open3DConfig.cmake file does not set Open3D_INCLUDE_DIRS correctly because the process of finding dependencies overrides PACKAGE_PREFIX_DIR. The proposed fix is to re-compute PACKAGE_PREFIX_DIR before defining Open3D_INCLUDE_DIRS.

[update-docs]

Thanks for submitting this pull request! The maintainers of this repository would appreciate if you could update the CHANGELOG.md based on your changes.