[codethinki]

Dependencies that install no CMake package (for example gtl, headers only) need a Find<Package>.cmake. Until now a package built with cth_cmake could find such a module during its own build, but the generated Config.cmake could not: find_dependency(<Package> MODULE) failed for every consumer of the installed package unless the consumer provided the module itself.

cth_pkg_add_find_modules(<files>...):

• registers the find modules and adds their directories to CMAKE_MODULE_PATH of the calling directory, so a following cth_pkg_target_find_package(<target> <Package> MODULE ...) finds them in the build
• cth_create_package() installs them next to the generated Config.cmake
• the generated Config.cmake appends its own directory to CMAKE_MODULE_PATH before the find_dependency blocks (a consumer's own module wins) and restores the consumer's module path at its end
• packages without find modules generate the same config as before

Tested locally with a producer that packages a target depending on a header only dummy library through Finddummy.cmake, and a consumer of the installed package: the consumer finds the dependency, builds and runs, and its CMAKE_MODULE_PATH is unchanged after find_package. Without the installed module the consumer fails with "dependency missing: find_package(dummy MODULE REQUIRED) failed". The CI examples do not install or consume packages, so this is not covered there.

Needed for gtl as a dependency of cth.