Find package fail when installed using submodule spdlog

[guangie88]

Somewhat related PR and issue: #21, #22

If spdlog_setup was installed with its spdlog found using find_package (i.e. no submodule spdlog), creating a dummy set-up to find_package(spdlog_setup) works perfectly after #22.

However using submodule causes the spdlog::spdlog import to fail using the dummy set-up:

CMake Error at CMakeLists.txt:5 (find_package):
  Found package configuration file:

    /usr/local/lib/cmake/spdlog_setup/spdlog_setup-config.cmake

  but it set spdlog_setup_FOUND to FALSE so package "spdlog_setup" is
  considered to be NOT FOUND.  Reason given by package:

  The following imported targets are referenced, but are missing:
  spdlog::spdlog