* Add a Nix expression search path feature. Paths between angle

brackets, e.g.

    import <nixpkgs/pkgs/lib>

  are resolved by looking them up relative to the elements listed in
  the search path.  This allows us to get rid of hacks like

    import "${builtins.getEnv "NIXPKGS_ALL"}/pkgs/lib"

  The search path can be specified through the ‘-I’ command-line flag
  and through the colon-separated ‘NIX_PATH’ environment variable,
  e.g.,

    $ nix-build -I /etc/nixos ...

  If a file is not found in the search path, an error message is
  lazily thrown.

src/libexpr/eval.cc

@@ -181,6 +181,12 @@ EvalState::EvalState()
         gcInitialised = true;
     }
 #endif
+
+    /* Initialise the Nix expression search path. */
+    searchPathInsertionPoint = searchPath.end();
+    Strings paths = tokenizeString(getEnv("NIX_PATH", ""), ":");
+    foreach (Strings::iterator, i, paths) addToSearchPath(*i);
+    searchPathInsertionPoint = searchPath.begin();
 }