nix: New installables syntax

The general syntax for an installable is now
<flakeref>:<attrpath>. The attrpath is relative to the flake's
'provides.packages' or 'provides' if the former doesn't yield a
result. E.g.

  $ nix build nixpkgs:hello

is equivalent to

  $ nix build nixpkgs:packages.hello

Also, '<flakeref>:' can be omitted, in which case it defaults to
'nixpkgs', e.g.

  $ nix build hello

src/libexpr/primops/flake.hh

@@ -23,7 +23,7 @@ Path getUserRegistryPath();
 
 Value * makeFlakeRegistryValue(EvalState & state);
 
-Value * makeFlakeValue(EvalState & state, std::string flakeUri, Value & v);
+Value * makeFlakeValue(EvalState & state, const FlakeRef & flakeRef, bool impureTopRef, Value & v);
 
 std::shared_ptr<FlakeRegistry> readRegistry(const Path &);