Fix exported CMake config

[dg0yt]

User projects may call 'find_package(c-ares)' multiple times, (e.g. via dependencies), but targets must be created only once. Otherwise users face this (microsoft/vcpkg#24367):

  add_library cannot create imported target "c-ares::cares_static" because
  another target with the same name already exists.
Call Stack (most recent call first):
  /home/xxxx/vcpkg/installed/x64-linux/share/c-ares/c-ares-config.cmake:29 (add_library)

In addition, shared and static target must be treated independently, to match the capabilites of the build system.