Dont look for spdlog if spdlog::spdlog is already defined

[jjcasmar]

If a project fetches spdlog from sources, spdlog::spdlog target will be defined, but a call to find_package(spdlog REQUIRED) will fail.

If that same project also fetches spdlog_setup from sources, it will fail, as spdlog_setup will do a call to find_package(spdlog REQUIRED) even though spdlog::spdlog already exists.

This patch adds a check before trying to look for spdlog.