* Refactoring: Get rid of a few subdirectories in corepkgs/, and some

other simplifications.
* Use <nix/...> to locate the corepkgs.  This allows them to be
  overriden through $NIX_PATH.
* Use bash's pipefail option in the NAR builder so that we don't need
  to create a temporary file.

corepkgs/buildenv.nix

@@ -0,0 +1,21 @@
+with import <nix/config.nix>;
+
+{ system, derivations, manifest }:
+
+derivation { 
+  name = "user-environment";
+  system = system;
+  builder = perl;
+  args = [ "-w" ./buildenv.pl ];
+  
+  manifest = manifest;
+
+  # !!! grmbl, need structured data for passing this in a clean way.
+  paths = derivations;
+  active = map (x: if x ? meta && x.meta ? active then x.meta.active else "true") derivations;
+  priority = map (x: if x ? meta && x.meta ? priority then x.meta.priority else "5") derivations;
+
+  # Building user environments remotely just causes huge amounts of
+  # network traffic, so don't do that.
+  preferLocalBuild = true;
+}