Wrokspaces not calling system_requirements

Hi !
I Have been struggling for days to find a way of using distribution packages when available instead of building all from conan sources packages. I use conan this way :
* include conanbuildinfo.cmake in my cmakelists.txt
* resolve dependencies using find_package generator + cmake find_package

Our internal packages are resolved as conan packages
Third party libraries are resolved as system dependencies if possible and as conan packages as fallback. CMake is using find_package in both cases

To simplify the mechanism on conan side, I created a python wrapper around conan to add a method
requires_package(dict)
where dict is pairs of distro/windows_subsystem/'conan' => package_name
it calls package tool install during system_requirements if the distro has an associated package
otherwise it will fallback to conan package with a warning.

So far, so good, it works like a charm on debian/arch/msys2 for a simple project.
But if i use the workspace feature, system_requirements() is not called and the build fail when cmake is calling the find_package macro as package has not been installed