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 fixes a build error occurring when building a configuration
having fontconfig enabled and `home.packages` only containing one
package installing things to `/lib`.
Also adds a number of test cases to verify the fontconfig cache
generation functionality.
Fixes#703
