Work on Values instead of Exprs

This prevents some duplicate evaluation in nix-env and
nix-instantiate.

Also, when traversing ~/.nix-defexpr, only read regular files with the
extension .nix.  Previously it was reading files like
.../channels/binary-caches/<name>.  The only reason this didn't cause
problems is pure luck (namely, <name> shadows an actual Nix
expression, the binary-caches files happen to be syntactically valid
Nix expressions, and we iterate over the directory contents in just
the right order).

src/libexpr/attr-path.hh

@@ -7,7 +7,7 @@
 
 namespace nix {
 
-void findAlongAttrPath(EvalState & state, const string & attrPath,
-    Bindings & autoArgs, Expr * e, Value & v);
+Value * findAlongAttrPath(EvalState & state, const string & attrPath,
+    Bindings & autoArgs, Value & vIn);
 
 }