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.
bacon reads its preferences from a different directory on Darwin.
Fix the path to read out of:
~/Library/Application\ Support/org.dystroy.bacon/prefs.toml
Linux behavior should be unchanged.
