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.
Add an extra option,
accounts.contact.accounts.<name>.khard.defaultCollection
to hardcode a subdirectory for khard to function as intended. Khard
expects to be given a collection directory directly, from which there
can be multiple of in a vdir. This contrasts khal or vdirsyncer which
support recursive search.
Fixes#4531