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.
Previously, lines in .zshrc were added with quotes in keys
(e.g. ZSH_HIGHLIGHT_STYLES['comment']='fg=#6c6c6c'). However, zsh
considered these quotes to be part of the key, so the "comment" key
remained unchanged.