Fix package-discovery CMake using incomplete install prefix #520
Description
CMake package-discovery tests incorrectly treat the install prefix as “installed” when stale ChronologConfig.cmake / chronolog.pc exist but libchronolog_client.so (and headers) are missing (mixed/partial install state). This makes find_package(Chronolog) link against install_dir/... and fails during configure, blocking make install.
Package-discovery should only run against a fully installed package (or be moved to CTest / run post-install) and otherwise fall back to the build tree.