First, we go through every environment variable key and record all cases
where there are reference to other variables / dependencies.
We track two sets of variables - resolved and yet-to-be-resolved.
We pass over a list of variables over and over again and when all
variable's dependencies were resolved during previous passes we perform
a replacement for that variable, too.
Over time the size of `toResolve` set should go down to zero, however
circular dependencies may prevent that. We track the size of `toResolve`
between iterations to avoid infinite looping.
At the end we produce an object of the same size and shape as
the original, but with the values replace with resolved versions.
