Merkle trees and build systems

For apt, the setting APT_CONFIG environment variable can entirely separate an apt config from the system config. Take a look at how chdist (from devscripts) or apt-venv do it. Unfortunately there isn't yet a way to tell a dpkg binary to not look at the system config with an equivalent DPKG_CONFIG variable, so things like mmdebstrap's sub-Essential mode break if you have packages that install dpkg hooks.