This is the first in a series of commits for refactoring my
configuration to make use of jwiegley's use-package.
In the previous layout of the configuration there were some questions
around what settings go into which file, but in the end it is all just
related to which packages are being configured (besides settings
related to global Emacs behaviour).
This commit introduces use-package forms for all currently used
packages (which are still installed via Nix, not via package.el) but
does not yet clean up the rest of the configuration in a suitable way.
Note that this version introduces a bug in which the configuration of
telephone line is not correctly initialised after package setup.