Add a `depends_on` directive

[vkarak]

I suggest adding a depends_on() directive (not built-in) that will act in the class body. This could essentially simply store the arguments in a special internal variable, that would then call obj.depends_on() during the variable injection. This way, we eliminate the need to define depends_on() in the __init__() method or in a post-init hook, as suggested by #1863.

@jjotero What do you think?

[vkarak]

Closing, see discussion in #1868.