cmake export configuration: allow repeating find_package(jsoncpp) calls (#1491)

In jsoncpp-namspaced-targets.cmake, it creates JsonCpp::JsonCpp imported
library without first checking whether it was already created by former
call to find_package(JsonCpp).  As CMake allows repeated call to
find_package(), the error of "another target with the same name already
exists" should be fixed.

Co-authored-by: xiazuoling.xzl <xiazuoling.xzl@alibaba-inc.com>
Co-authored-by: Jordan Bayles <bayles.jordan@gmail.com>

Author: 3 people