emacs-guix does not see packages outside of `gnu/packages` available via external channels
Because we interact with guix using a normal guile repl instead of guix repl
, fold-packages
only folds over the packages in the default guix channel. Best would be to automagically find all packages from all channels, but being able to configure which 'extra' package directories to look at from the emacs side would already be a nice feature.