Subproject git modules not cloned recursively

When running cmake, a subproject module is initialized and cloned, but this project modules are not cloned, however, causing the build to fail.

CMake/GitExternal.cmake

Line 114 in 935807c

execute_process(COMMAND "${GIT_EXECUTABLE}" submodule update --recursive

The reason is that the call to git submodule init is not recursive, not registering the paths of the subproject module's modules, thus when the git submodule update --recursive call is executed, these non registered paths are not cloned.
Executing these 3 commands manually results in the same behaviour.

The test case is Brion https://github.com/BlueBrain/Brion
The scenario where this happens occurs when dealing with its sub module "libsonata". The structure is:

- Brion
    - libsonata
        - HighFive
        - fmt

If Brion is the subproject, when building, libsonata will be cloned, but HighFive and fmt (which are git submodules) will not be cloned.

adding --init to the line pointed above solves the problem, although it clones again CMake/common.

This can be fixed manually by running git submodule udpate --init in the problematic subprojects, however this cannot be applied to the CI jobs.