Allow bare flakerefs as installables

So now

  $ nix build blender-bin

works and builds the default package from that flake. You don't need
to add a colon at the end anymore.

src/libexpr/primops/flakeref.hh

@@ -180,4 +180,9 @@ struct FlakeRef
 
 std::ostream & operator << (std::ostream & str, const FlakeRef & flakeRef);
 
+MakeError(BadFlakeRef, Error);
+
+std::optional<FlakeRef> parseFlakeRef(
+    const std::string & uri, bool allowRelative = false);
+
 }