nix: Add --flake flag

This allows using an arbitrary "provides" attribute from the specified
flake. For example:

  nix build --flake nixpkgs packages.hello

(Maybe provides.packages should be used for consistency...)

src/libexpr/primops/flake.hh

@@ -5,6 +5,9 @@
 
 namespace nix {
 
+struct Value;
+class EvalState;
+
 struct FlakeRegistry
 {
     struct Entry
@@ -14,4 +17,8 @@ struct FlakeRegistry
     std::map<FlakeId, Entry> entries;
 };
 
+Value * makeFlakeRegistryValue(EvalState & state);
+
+Value * makeFlakeValue(EvalState & state, std::string flakeUri, Value & v);
+
 }