[C++][CMake] arrow_bundled_dependencies is non-accessible when used externally via FetchContent

[wgtmac]

Describe the bug, including details regarding any error messages, version, and platform.

arrow_bundled_dependencies is an IMPORTED library which links to arrow_bundled_dependencies_merge. When arrow-cpp is used externally (e.g. apache/iceberg-cpp#89) as a third-party dependency via CMake FetchContent, target arrow_bundled_dependencies is not accessible by the parent project (but arrow_bundled_dependencies_merge is). It is difficult for the parent project to locate the built product of arrow_bundled_dependencies_merge and install it.

Component(s)

C++, Packaging

[kou]

Issue resolved by pull request 46232
#46232