* nix-env: allow ~/.nix-defexpr to be a directory. If it is, then the

Nix expressions in that directory are combined into an attribute set
  {file1 = import file1; file2 = import file2; ...}, i.e. each Nix
  expression is an attribute with the file name as the attribute
  name.  Also recurses into directories.

* nix-env: removed the "--import" (-I) option which set the
  ~/.nix-defexpr symlink.

* nix-channel: don't use "nix-env --import", instead symlink
  ~/.nix-defexpr/channels.  So finally nix-channel --update doesn't
  override any default Nix expressions but combines with them.

  This means that you can have (say) a local Nixpkgs SVN tree and use
  it as a default for nix-env:

  $ ln -s .../path-to-nixpkgs-tree ~/.nix-defexpr/nixpkgs_svn

  and be subscribed to channels (including Nixpkgs) at the same time.
  (If there is any ambiguity, the -A flag can be used to
  disambiguate, e.g. "nix-env -i -A nixpkgs_svn.pan".)

src/libexpr/get-drvs.cc

@@ -171,8 +171,8 @@ static void getDerivations(EvalState & state, Expr e,
         ATermMap drvMap(ATgetLength(es));
         queryAllAttrs(e, drvMap);
 
-        /* !!! undocumented hackery to support
-           corepkgs/channels/unpack.sh. */
+        /* !!! undocumented hackery to support combining channels in
+           nix-env.cc. */
         Expr e2 = drvMap.get(toATerm("_combineChannels"));
         bool combineChannels = e2 && evalBool(state, e2);