statsd: Don't use <nixpkgs>

This was preventing the Nixpkgs channel from updating, since the
program indexer barfed on:

error: anonymous function at /nix/store/wdnwbh3kmf68nhqqp0khcyxbdbp43vg5-nixos-14.12.626.b0d594c/nixos/nixpkgs/pkgs/top-level/node-packages.nix:1:1 called without required argument ‘neededNatives’, at /data/releases/nixos/unstable-small/.tmp-nixos-16.03pre72946.c50d013-787/unpack/nixos-16.03pre72946.c50d013/lib/customisation.nix:56:12

because Nixpkgs 16.03 was importing files from Nixpkgs 14.12.

Also added some half-assed checks to detect this issue in the future.

pkgs/development/python-modules/blivet/default.nix

···
     six
   ];
 
-  # Tests are in <nixpkgs/nixos/tests/blivet.nix>.
+  # Tests are in nixos/tests/blivet.nix.
   doCheck = false;
 
   meta = with stdenv.lib; {

pkgs/tools/networking/statsd/default.nix

···
 
 let
   self = recurseIntoAttrs (
-    callPackage <nixpkgs/pkgs/top-level/node-packages.nix> {
+    callPackage ../../../top-level/node-packages.nix {
       inherit nodejs self;
       generated = callPackage ./node-packages.nix { inherit self; };
       overrides = {

pkgs/top-level/make-tarball.nix

···
   checkPhase = ''
     export NIX_DB_DIR=$TMPDIR
     export NIX_STATE_DIR=$TMPDIR
+    export NIX_PATH=nixpkgs=$TMPDIR/barf.nix
     nix-store --init
 
+    echo 'abort "Illegal use of <nixpkgs> in Nixpkgs."' > $TMPDIR/barf.nix
+
+    # Make sure that Nixpkgs does not use <nixpkgs>
+    if grep -r '<nixpkgs\/' pkgs; then
+        echo "Nixpkgs is not allowed to use <nixpkgs> to refer to itself."
+        exit 1
+    fi
+
     # Make sure that derivation paths do not depend on the Nixpkgs path.
     mkdir $TMPDIR/foo
     ln -s $(readlink -f .) $TMPDIR/foo/bar
-    p1=$(nix-instantiate pkgs/top-level/all-packages.nix --dry-run -A firefox)
+    p1=$(nix-instantiate pkgs/top-level/all-packages.nix --dry-run -A firefox --show-trace)
     p2=$(nix-instantiate $TMPDIR/foo/bar/pkgs/top-level/all-packages.nix --dry-run -A firefox)
     if [ "$p1" != "$p2" ]; then
         echo "Nixpkgs evaluation depends on Nixpkgs path ($p1 vs $p2)!"
···
 
     # Check that all-packages.nix evaluates on a number of platforms without any warnings.
     for platform in i686-linux x86_64-linux x86_64-darwin; do
-        header "checking pkgs/top-level/all-packages.nix on $platform"
+        header "checking Nixpkgs on $platform"
 
-        NIXPKGS_ALLOW_BROKEN=1 nix-env -f pkgs/top-level/all-packages.nix \
+        NIXPKGS_ALLOW_BROKEN=1 nix-env -f . \
             --show-trace --argstr system "$platform" \
             -qa --drv-path --system-filter \* --system 2>&1 >/dev/null | tee eval-warnings.log
 
         if [ -s eval-warnings.log ]; then
-            echo "pkgs/top-level/all-packages.nix on $platform evaluated with warnings, aborting"
+            echo "Nixpkgs on $platform evaluated with warnings, aborting"
             exit 1
         fi
         rm eval-warnings.log
 
-        NIXPKGS_ALLOW_BROKEN=1 nix-env -f pkgs/top-level/all-packages.nix \
+        NIXPKGS_ALLOW_BROKEN=1 nix-env -f . \
             --show-trace --argstr system "$platform" \
             -qa --drv-path --system-filter \* --system --meta --xml > /dev/null
         stopNest