Do not generate non-relocatable files in yarp_configure_plugins_installation

[traversaro]

Is your feature request related to a problem? Please describe.
The yarp_configure_plugins_installation macro (that is also called in yarp_configure_external_installation) installs a non-relocatable configuration file to correctly find devices installed by downstream packages, see:

• https://github.com/robotology/yarp/blob/v3.4.1/cmake/YarpInstallationHelpers.cmake#L77
• https://github.com/robotology/yarp/blob/v3.4.1/src/libYARP_os/src/yarp/os/YarpPlugin.cpp#L96

This is problematic as it make YARP and downstream packages that depend on YARP and use yarp_configure_plugins_installation or yarp_configure_external_installation to install YARP devices non-relocatable packages, i.e. packages that will not work if you change the installation directory path after the installation, or if installed via DESTDIR .

Describe the solution you'd like
A possible solution is to modify the .ini generation logic in https://github.com/robotology/yarp/blob/v3.4.1/cmake/YarpInstallationHelpers.cmake#L77 to create a relative_path argument that specifies the relative location of the plugins directory, instead of specifying the absolute path with path.
Furthermore, it would be necessary to modify the C++ logic that reads this files in https://github.com/robotology/yarp/blob/v3.4.1/src/libYARP_os/src/yarp/os/YarpPlugin.cpp#L96 to interpreter correctly this new relative_path argument.

Describe alternatives you've considered
A possible alternative is to have some custom logic in the packaging, such as done in https://github.com/robotology/icub-packaging/blob/5768d6c1768a26c583ecb6866b67494fe37841c5/linux/build-iCub-packages.sh#L404 or how conda does automatically (https://docs.conda.io/projects/conda-build/en/latest/resources/make-relocatable.html), but this solution need to be ad-hoc for each packaging system, and do not work for example if you want to just distribute installation prefixes simply as .zip directories.

[drdanz]

Partially related: #522