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.
The way the `bat` module is currently written makes it essentially
impossible to use themes and syntaxes without IFD, since you must
provide the contents as string, instead of just giving a path to be
linked.
With this change, setting themes/syntaxes by-string will start issuing
warnings, and a new attribute model is added, lightly inspired by how
`programs.zsh.plugins` avoided this issue.