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.
Specifically, allow variable expansion for the key codes by switching
from single to double quotes.
This also adds a helpful suggestion to descriptions. Taken from the
project's README, see
4abed97b6e/README.md (L71-L74))
It's pretty common to need multiple bindings to
history-substring-search, since different terminals will send different
keys for up/down.
This does not break back-compatibility, and introduces a new test
