Change FindGzAssimp to use find_package

[luca-della-vedova]

🦟 Bug fix

Summary

The previous implementation of FindGzAssimp had issues on Windows, specifically on Windows it seems that it is necessary to use find_package instead of pkg-config, i.e. vcpkg prints the following:

The package assimp provides CMake targets:

    find_package(assimp CONFIG REQUIRED)
    target_link_libraries(main PRIVATE assimp::assimp)

but using find_package sets global variables that break downstream users that reimplement their own version of FindAssimp such as dartsim.

For these reasons the implementation now uses find_package but clears the cache from the assimp variables and only uses the GzAssimp namespaced variables. Downstream behavior is now unchanged.

Tested with the gz-common PR to make sure CI is ✔️

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[chapulina]

Merging without the Windows CI to clear up the build farm. Windows CI on this repo shouldn't be catching any issues with this PR anyway.

[chapulina]

Nightlies:

• Focal
• Jammy