Feature request: allow packages to append to CMAKE_MODULE_PATH from their cmake config file

[scpeters]

Desired behavior

gz-cmake3 installs many cmake files and adds the containing folder to the CMAKE_MODULE_PATH so that packages that find_package(gz-cmake3) will be able to use those cmake files.

• https://github.com/gazebosim/gz-cmake/blob/gz-cmake3_3.0.1/config/gz-cmake-config.cmake.in#L61

We would like to install cmake files from other packages as well (such as gz-physics) and append the path of the installed folder to the CMAKE_MODULE_PATH. We could expose this functionality as a flag or parameter to the gz_create_core_library and gz_add_component cmake functions, since that is where the cmake config templates are instantiated.

• _gz_create_cmake_package function in GzPackaging.cmake is called by gz_create_core_library and gz_add_component in GzUtils.cmake.

Alternatives considered

Unconditionally add a specific path for each package to the CMAKE_MODULE_PATH so it doesn't have to be configured. This may pollute the CMAKE_MODULE_PATH, so it would be cleaner to opt in to the functionality.

Implementation suggestion

Additional context

[mjcarroll]

I'm going to go ahead and take this on as part of gazebosim/gz-msgs#113.

I think rather than appending to the CMAKE_MODULE_PATH, I may take the approach that ament does and allow the user to generate a gz-package<VER>-extras.cmake file that can provide hooks into other installed cmake files.