cmake: find_package(RabbitMQ) instead of just checking its header

cmake: find_package(RabbitMQ) instead of just checking its header

rabbitmq-c is not a header-only library. so we need to ensure that the
library is also around before linking against it. in this change:

* move the detection down to where librabbitmq is used
* add FindRabbitMQ.cmake for find_package(RabbitMQ)
* s/rabbitmq/RabbitMQ::RabbitMQ/ as the latter is an alias library
target, which has more information attached to it.

Signed-off-by: Kefu Chai <kchai@redhat.com>