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/sowing.nix

@@ -0,0 +1,23 @@
+{ stdenv, fetchFromBitbucket, ghostscript }:
+
+stdenv.mkDerivation rec {
+  version = "1.1.25-p1";
+  name = "sowing-${version}";
+
+  src = fetchFromBitbucket {
+    owner = "petsc";
+    repo = "pkg-sowing";
+    rev = "v${version}";
+    sha256 = "0mpvq9jdavps5ljd2l1qngsyr96g2mp4ic21zf2bf80qvfjyanyw";
+  };
+
+
+  buildInputs = [
+    ghostscript
+  ];
+
+  meta = with stdenv.lib; {
+    homepage = "https://bitbucket.org/petsc/pkg-sowing";
+    description = "Tools that are part of the petsc program development and maintenance environment.";
+  };
+}