[BUG] rapids_cpm_find(... DOWNLOAD_ONLY ON) writes an empty find_package redirect #1113

Describe the bug
Since #1089, rapids_cpm_find translates DOWNLOAD_ONLY into SOURCE_SUBDIR .rapids-cmake-download-only/<name>. CPM only skips writing its find_package redirect when DOWNLOAD_ONLY is set, so every download-only package now gets an empty <name>-config.cmake stub in CMAKE_FIND_PACKAGE_REDIRECTS_DIR. Every later find_package(<name>) in the build reports the package as found but defines no targets, including calls made by other projects' package configs.

This breaks Velox with cuDF enabled: cuDF fetches xxhash with DOWNLOAD_ONLY, and FBThrift's config later calls find_package(Xxhash MODULE) expecting its own FindXxhash.cmake to create Xxhash::xxhash. The stub answers instead and generation fails (facebookincubator/velox#19909). The same applies to cuDF's dlpack, and rapids_cpm_bs_thread_pool (which passes the placeholder SOURCE_SUBDIR directly) also writes a bs_thread_pool stub.

Steps/Code to reproduce bug

include(${rapids-cmake-dir}/cpm/init.cmake)
include(${rapids-cmake-dir}/cpm/find.cmake)
rapids_cpm_init()

set(CPM_DOWNLOAD_dlpack ON)
rapids_cpm_find(dlpack 1.3
                CPM_ARGS
                GIT_REPOSITORY https://github.com/dmlc/dlpack.git
                GIT_TAG v1.3
                DOWNLOAD_ONLY ON)

# Exists on 26.10+, absent before #1089
message(STATUS "${CMAKE_FIND_PACKAGE_REDIRECTS_DIR}/dlpack-config.cmake")

Expected behavior
DOWNLOAD_ONLY packages don't register a find_package redirect, as before #1089. A later find_package(<name> MODULE) should still reach the caller's own Find<name>.cmake.

Environment details (please complete the following information):

• rapids-cmake 26.10 and main
• CMake 4.x

Additional context
xref NVIDIA/cudf#25124, which works around this for xxhash only. rapids_cpm_cccl already suppresses the redirect the same way (#997).