The mock pkgs set passed `{}` as the value for unknown args; if a
package def tried to evaluate one, this can raise an error that can't be
caught. Replacing the default value with `throw ""` enables bailing out
if one of the fake args is accessed.
Since evaluating pkgs is expensive, this speeds up evaluation of flakes
with `packages.default` by attempting to get the name using a pkgs set
containing only mocked builders that just return the name.
