refactor Emacs packages'/modes' builders

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.

pkgs/build-support/emacs/generic.nix

@@ -0,0 +1,41 @@
+# generic builder for Emacs packages
+
+{ lib, stdenv, emacs, texinfo }:
+
+with lib;
+
+{ pname
+, version ? null
+
+, buildInputs ? []
+, packageRequires ? []
+
+, meta ? {}
+
+, ...
+}@args:
+
+let
+  defaultMeta = {
+    broken = false;
+    platforms = emacs.meta.platforms;
+  };
+in
+
+stdenv.mkDerivation ({
+  name = "emacs-${pname}${optionalString (version != null) "-${version}"}";
+
+  buildInputs = [emacs texinfo] ++ packageRequires ++ buildInputs;
+  propagatedBuildInputs = packageRequires;
+  propagatedUserEnvPkgs = packageRequires;
+
+  setupHook = ./setup-hook.sh;
+
+  doCheck = false;
+
+  meta = defaultMeta // meta;
+}
+
+// removeAttrs args [ "buildInputs" "packageRequires"
+                      "meta"
+                    ])