[libpng] Fix find_package debug libraries

[dg0yt]

Describe the pull request

• What does your PR fix?

  • Fix [libpng] CMake find module finds debug library in release on linux #24900
  • [cmake-user] Extend check for release vs. debug libs

• Which triplets are supported/not supported? Have you updated the CI baseline?

  unchanged, no

• Does your PR follow the maintainer guide?

  yes

• If you have added/updated a port: Have you run ./vcpkg x-add-version --all and committed the result?

  yes

[ras0219-msft]

Thanks a bunch for the fix and test improvement!