emacs.pkgs.elisp-ffi: move to a dedicated directory

There is no tangible reason to pollute a file like manual-packages with full
descriptions of expressions. `callPackage` exists for a reason :)

pkgs/applications/editors/emacs/elisp-packages/elisp-ffi/default.nix

@@ -0,0 +1,47 @@
+{ lib
+, melpaBuild
+, fetchFromGitHub
+, pkg-config
+, libffi
+, writeText
+}:
+
+let
+  rev = "da37c516a0e59bdce63fb2dc006a231dee62a1d9";
+in melpaBuild {
+  pname = "elisp-ffi";
+  version = "20170518.0";
+
+  commit = rev;
+
+  src = fetchFromGitHub {
+    owner = "skeeto";
+    repo = "elisp-ffi";
+    inherit rev;
+    sha256 = "sha256-StOezQEnNTjRmjY02ub5FRh59aL6gWfw+qgboz0wF94=";
+  };
+
+  nativeBuildInputs = [ pkg-config ];
+
+  buildInputs = [ libffi ];
+
+  preBuild = ''
+    mv ffi.el elisp-ffi.el
+    make
+ '';
+
+  recipe = writeText "recipe" ''
+   (elisp-ffi :repo "skeeto/elisp-ffi" :fetcher github)
+  '';
+
+  meta = {
+    description = "Emacs Lisp Foreign Function Interface";
+    longDescription = ''
+        This library provides an FFI for Emacs Lisp so that Emacs
+        programs can invoke functions in native libraries. It works by
+        driving a subprocess to do the heavy lifting, passing result
+        values on to Emacs.
+      '';
+    license = lib.licenses.publicDomain;
+  };
+}

pkgs/applications/editors/emacs/elisp-packages/manual-packages.nix

@@ -2,47 +2,7 @@
 
 self: with self; {
 
-  elisp-ffi = let
-    rev = "da37c516a0e59bdce63fb2dc006a231dee62a1d9";
-  in melpaBuild {
-    pname = "elisp-ffi";
-    version = "20170518.0";
-
-    commit = rev;
-
-    src = pkgs.fetchFromGitHub {
-      owner = "skeeto";
-      repo = "elisp-ffi";
-      inherit rev;
-      sha256 = "sha256-StOezQEnNTjRmjY02ub5FRh59aL6gWfw+qgboz0wF94=";
-    };
-
-    nativeBuildInputs = [ pkgs.pkg-config ];
-
-    buildInputs = [ pkgs.libffi ];
-
-    preBuild = ''
-      mv ffi.el elisp-ffi.el
-      make
-    '';
-
-    recipe = pkgs.writeText "recipe" ''
-      (elisp-ffi
-      :repo "skeeto/elisp-ffi"
-      :fetcher github)
-    '';
-
-    meta = {
-      description = "Emacs Lisp Foreign Function Interface";
-      longDescription = ''
-        This library provides an FFI for Emacs Lisp so that Emacs
-        programs can invoke functions in native libraries. It works by
-        driving a subprocess to do the heavy lifting, passing result
-        values on to Emacs.
-      '';
-      license = lib.licenses.publicDomain;
-    };
-  };
+  elisp-ffi = callPackage ./elisp-ffi { };
 
   ghc-mod = melpaBuild {
     pname = "ghc";