No real function change here, but this updates the trivial and melpa builders to
be formatted more consistently with the rest of the builders, and swaps
`eval "$preBuild"` for the more standard `runHook preBuild`.
This simplifies melpa builder by merging with it my old emacs modes builder,
adds better instructions and support for overrides in emacs-packages.nix,
and renames some emacs-related stuff in all-packages.nix for sanity reasons.
I declare this backwards compatible since direct uses of emacsPackages in
configuration.nix are very unlikely.
