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/nix/installables.cc

@@ -217,25 +217,20 @@ void makeFlakeClosureGCRoot(Store & store,
     assert(store.isValidPath(resFlake.flake.sourceInfo.storePath));
     closure.insert(resFlake.flake.sourceInfo.storePath);
 
-    std::queue<std::reference_wrapper<const flake::FlakeInputs>> queue;
+    std::queue<std::reference_wrapper<const flake::LockedInputs>> queue;
     queue.push(resFlake.lockFile);
 
     while (!queue.empty()) {
-        const flake::FlakeInputs & flake = queue.front();
+        const flake::LockedInputs & flake = queue.front();
         queue.pop();
         /* Note: due to lazy fetching, these paths might not exist
            yet. */
-        for (auto & dep : flake.flakeInputs) {
+        for (auto & dep : flake.inputs) {
             auto path = dep.second.computeStorePath(store);
             if (store.isValidPath(path))
                 closure.insert(path);
             queue.push(dep.second);
         }
-        for (auto & dep : flake.nonFlakeInputs) {
-            auto path = dep.second.computeStorePath(store);
-            if (store.isValidPath(path))
-                closure.insert(path);
-        }
     }
 
     if (closure.empty()) return;