Emacs packages are commonly distributed as single .el files. This
unpackCmd handles them correctly and sets up sourceRoot. Other sources
are treated in the default manner.
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.
