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/flake/flake.hh

@@ -58,14 +58,20 @@ struct SourceInfo
     SourceInfo(const FlakeRef & resolvRef) : resolvedRef(resolvRef) {};
 };
 
+struct FlakeInput
+{
+    FlakeRef ref;
+    bool isFlake = true;
+    FlakeInput(const FlakeRef & ref) : ref(ref) {};
+};
+
 struct Flake
 {
     FlakeId id;
     FlakeRef originalRef;
     std::string description;
     SourceInfo sourceInfo;
-    std::vector<FlakeRef> inputs;
-    std::map<FlakeAlias, FlakeRef> nonFlakeInputs;
+    std::map<FlakeId, FlakeInput> inputs;
     Value * vOutputs; // FIXME: gc
     unsigned int edition;
 
@@ -73,14 +79,6 @@ struct Flake
         : originalRef(origRef), sourceInfo(sourceInfo) {};
 };
 
-struct NonFlake
-{
-    FlakeRef originalRef;
-    SourceInfo sourceInfo;
-    NonFlake(const FlakeRef & origRef, const SourceInfo & sourceInfo)
-        : originalRef(origRef), sourceInfo(sourceInfo) {};
-};
-
 Flake getFlake(EvalState &, const FlakeRef &);
 
 /* If 'allowLookup' is true, then resolve 'flakeRef' using the
@@ -108,7 +106,7 @@ ResolvedFlake resolveFlake(EvalState &, const FlakeRef &, HandleLockFile);
 
 void callFlake(EvalState & state,
     const Flake & flake,
-    const FlakeInputs & inputs,
+    const LockedInputs & inputs,
     Value & v);
 
 void callFlake(EvalState & state,