We previously used a loader -> BuildConfig flow to load configuration.
This commit changes most (but not all yet) flows to use the new
graph-based approach. Instead of simply recursively loading the modules,
we now need to take a stepped approach:
1. Load the root module
2. Collect the variables and their values
3. Build the configuration with the graph-based approach
Because this approach relies on different parts from different packages,
it can't easliy be done within the `configload` package. So, now we do
most of in the backend or command.
