When building a package from a Melpa recipe file, get the Emacs package
name from the recipe. Nix is more restrictive about packages names than
Emacs, so the Nix name for a package is sometimes different.
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.
