revert to "build plan" in overview diagram
revert to "build plan" in overview diagram

this displays correct composition again. build inputs and build results
are not part of build plans in terms of data objects.

also this is a much less complicated setup. this will be the first
impression of architecture, and we want to get it right.

NixOS/nix,NixOS/nix,NixOS/nix,NixOS/nix,NixOS/nix