CMakePackageConfigHelpers: Restore undocumented PACKAGE_PREFIX_DIR variable Revert commit 6ddf8712cd (CMakePackageConfigHelpers: Use unique variable name for pkg prefix, 2024-03-30, v3.29.1~8^2). It regressed existing projects that rely on the undocumented `PACKAGE_PREFIX_DIR` variable in package configuration files generated by `CMakePackageConfigHelpers`. Fixes: #25873 Fixes: #25885 Issue: #25827