Turn flake inputs into an attrset

Instead of a list, inputs are now an attrset like

  inputs = {
    nixpkgs.uri = github:NixOS/nixpkgs;
  };

If 'uri' is omitted, than the flake is a lookup in the flake registry, e.g.

  inputs = {
    nixpkgs = {};
  };

but in that case, you can also just omit the input altogether and
specify it as an argument to the 'outputs' function, as in

  outputs = { self, nixpkgs }: ...

This also gets rid of 'nonFlakeInputs', which are now just a special
kind of input that have a 'flake = false' attribute, e.g.

  inputs = {
    someRepo = {
      uri = github:example/repo;
      flake = false;
    };
  };

src/libexpr/primops/fetchGit.cc

@@ -31,7 +31,6 @@ GitInfo exportGit(ref<Store> store, std::string uri,
     // or revision is given, then allow the use of an unclean working
     // tree.
     if (!ref && !rev && isLocal) {
-
         bool clean = true;
 
         try {