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.
Emacs will call package-initialize itself, if required, or the user will
call it in their initialization file. There is no reason to call it in
the wrapper and doing so only increases start-up time.
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.
