firedrake: Moved to new overlay system and shared sub-scope

Firedrake contains a ton of inter-connected dependencies, many of
which are either firedrake specific or forked versions of upstream
packages.  Cleaner to create a firedrake sub-scope for all these.

pkgs/firedrake/default.nix

@@ -0,0 +1,49 @@
+# This just brings in everything.
+#
+# This isn't really informative or overridable in any practical sense.  Could put the closure of the dependencies
+# of all the packages declared here instead.  This would be overridable/informative, but a bigger pain to maintain.
+#
+# Likely this should be added into the python packages set instead of dropped on the top-level.
+#
+{ pkgs }:
+
+let
+  self =
+    let
+      # Top-level scope extended with this set of packages.
+      #
+      # Possibly it would make more sense to use the python-level scope instead?
+      #
+      callPackage = pkgs.newScope self;
+
+    in
+      with self; {
+        inherit callPackages;
+
+        # Python to use for all this stuff (much of it is python)
+        python         = pkgs.python27;
+        pythonPackages = python.pkgs;
+
+        # Forked and unavailable packages required by firedrake and its dependencies
+        ufl       = callPackage ./ufl.nix       { };
+        fiat      = callPackage ./fiat.nix      { };
+        finat     = callPackage ./finat.nix     { };
+        tsfc      = callPackage ./tsfc.nix      { };
+        pyop2     = callPackage ./pyop2.nix     { };
+        petsc     = callPackage ./petsc.nix     { };
+        petsc4py  = callPackage ./petsc4py.nix  { };
+        coffee    = callPackage ./coffee.nix    { };
+        firedrake = callPackage ./firedrake.nix { };
+        pulp      = callPackage ./pulp.nix      { };
+        sowing    = callPackage ./sowing.nix    { };
+        metis     = callPackage ./metis.nix     { };
+        hypre     = callPackage ./hypre.nix     { };
+        parmetis  = callPackage ./parmetis.nix  { };
+        exodus    = callPackage ./exodus.nix    { };
+
+        # Incompatible version (something with communicator/datatype cleanup callbacks?)
+        mpi4py    = callPackage ./mpi4py.nix { };
+      };
+
+in
+  self.firedrake