Improve base dependencies context

[sveetch]

Currently each dependency is stored as its own variable in context. It was simple enough but it growed a little and it is not easy to distinguish the ones for bireli, backend or frontend.

New variables _backend_stack and _frontend_stack have been added to resolve this but this not efficient.

A new simpler way have to be done. The best is just to replace current variables with a one for bireli, another for backend and one for frontend.

A search in template(s) for current variables have to be done before to replace them with their new names.

This should results to something like:

"_backend_stack": {
    "python": ">=3.10",
}

And in template files we would use it like {{ cookiecutter._backend_stack.python }}.

Finally, current sphinx directive to output the stack will have to be modified to work with this.