Fixes#10819. emacsWithPackages will know its own package set. This
requires it to be in a package set, rather than at the top level, so it
lives in emacsPackagesNg.
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.
