Add emacsWithPackagesFromPackageRequires

This provides a mechanism for creating an Emacs closure that contains
the runtime dependencies for a given Emacs package source file, by
inspecting its Package-Requires header.

packreq.nix

@@ -0,0 +1,26 @@
+/*
+Parse an emacs package file to derive packages from
+Package-Requires declarations.
+*/
+
+{ pkgs }:
+let
+  parse = pkgs.callPackage ./parse.nix { };
+in
+{ packageFile
+, extraEmacsPackages ? epkgs: [ ]
+, package ? pkgs.emacs
+, override ? (epkgs: epkgs)
+}:
+let
+  packages = parse.parsePackagesFromPackageRequires packageFile;
+  emacsPackages = pkgs.emacsPackagesGen package;
+  emacsWithPackages = emacsPackages.emacsWithPackages;
+in
+emacsWithPackages (epkgs:
+  let
+    overriden = override epkgs;
+    usePkgs = builtins.map (name: overriden.${name}) packages;
+    extraPkgs = extraEmacsPackages overriden;
+  in
+  [ overriden.use-package ] ++ usePkgs ++ extraPkgs)