arrow

ARROW-17565: [C++] Backward compatible ${PACKAGE}_shared CMake target isn't provided (#14003)

This is a follow-up of ARROW-12175 / #13892 .

We introduced `${PACKAGE}::` namespace to all exported CMake targets
such as `Arrow::arrow_shared` and `Arrow::arrow_static`, but we also
provided no-namespaced CMake targets such as `arrow_shared` and
`arrow_static` as aliases of namespaced CMake targets.

However, the logic to provide `arrow_shared` was buggy.


Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>