This reverts commits 95861b5d9f and
d2c014e1c7 (PR #6778), as they broke
Aerospace configuration loading. Whilst the generated TOML configuration
wasn't the best, it was completely valid, whereas after the
aforementioned PR it was no longer valid.
Instead of having to manually stub packages that should not be
downloaded we instead automatically stub all packages (except a small
list of whitelisted ones). Tests can re-introduce the real package by
using the `realPkgs` module argument.
