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.
Allows for the database path for mu to be configured. Useful for keeping
the maildir and mu xapian cache together without having to modify
XDG_CACHE_HOME. Add test to check for custom home setting.
Fixes#5534