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.
This adds support for configuring email accounts, with automatic smtp, imap,
sendmail (msmpt) and maildir (mbsync, offlineimap) setup in aerc,
via `accounts.email`.