[sdl2-mixer] Fix usage

[Thomas1664]

Describe the pull request

• What does your PR fix?

  Related: [sdl2-mixer] CMake find_package is incomplete #26064

[Cheney-W]

I think this PR is invalid, this issue #26064 is not a vcpkg issue. I have tested sdl2-mixer in my side and the original usage works fine.

[dg0yt]

The PR is valid even if it doesn't fix #26064. Neither users nor heuristics can figure out

target_link_libraries($<IF:$<TARGET_EXISTS:SDL2_mixer::SDL2_mixer>,SDL2_mixer::SDL2_mixer,SDL2_mixer::SDL2_mixer-static>)

so it takes an explicit usage file.

[dg0yt]

For consistency, I would suggest to add a new line after this line.

[Thomas1664]

Yes. We should fix the auto-generated usage as well.

[dg0yt]

Ah, I see, the # this is heuristically ... was added only a short time ago.

[Thomas1664]

@dg0yt I created microsoft/vcpkg-tool#653 . The usage generation looks weird: We search in every CMake file in share for add_library and add the name(s) of the library to the list of exported targets. However, CMake just searches for <namespace>/target-name-config.cmake and <namespace>/target-nameConfig.cmake. I think we should adopt this.

[dg0yt]

@Thomas1664 See #20190 for test port candidates and proposals for improvement (#20190 (comment)).

[Thomas1664]

Just for reference the suggested output is

Total elapsed time: 15.44 s

sdl2-mixer provides CMake targets:
    # this is heuristically generated, and may not be correct
    find_package(SDL2_mixer CONFIG REQUIRED)
    # note: 6 additional targets are not displayed.
    target_link_libraries(main PRIVATE FLAC::FLAC MPG123::mpg123 libxmp::libxmp tremor::tremor)

This is wrong because there aren't any sdl2-mixer libraries.