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/tsfc.nix

@@ -0,0 +1,30 @@
+{ lib, callPackage, fetchFromGitHub, pythonPackages
+, coffee, ufl, fiat, finat }:
+
+pythonPackages.buildPythonPackage rec {
+  version = "ee5670ab1ad90aaf8f550de01255c6752583255d";
+  name = "firedrake-tsfc-${version}";
+
+  src = fetchFromGitHub {
+    owner = "firedrakeproject";
+    repo = "tsfc";
+    rev = "${version}";
+    sha256 = "07v5v1nhcywn2rpdr0380ck120ppzlvpfknmjh1rss6xkzf2s9lf";
+  };
+
+  propagatedBuildInputs = [
+    pythonPackages.numpy
+    pythonPackages.singledispatch
+    pythonPackages.six
+    coffee
+    ufl
+    fiat
+    finat
+  ];
+
+  meta = with lib; {
+    homepage = "https://github.com/firedrakeproject/tsfc";
+    description = "Form compiler for the Firedrake project.";
+    license = licenses.gpl3;
+  };
+}