Change JSON derivation format in two ways

- Use canonical content address JSON format for floating content
  addressed derivation outputs

  This keeps it more consistent.

- Reorganize inputs into nested structure (`inputs.srcs` and
  `inputs.drvs`)

  This will allow for an easier to use, but less compact, alternative
  where `srcs` is just a list of derived paths.

  It also allows for other experiments for derivations with a different
  input structure, as I suspect will be needed for secure build traces.

tests/functional/dyn-drv/non-trivial.nix

@@ -51,10 +51,12 @@ builtins.outputOf
             "$word": "hello, from $word!",
             "PATH": ${builtins.toJSON path}
           },
-          "inputDrvs": {
-            $inputDrvs
+          "inputs": {
+            "drvs": {
+              $inputDrvs
+            },
+            "srcs": []
           },
-          "inputSrcs": [],
           "name": "build-$word",
           "outputs": {
             "out": {
@@ -63,7 +65,7 @@ builtins.outputOf
             }
           },
           "system": "${system}",
-          "version": 3
+          "version": 4
         }
       EOF
         drvPath=$(echo "$json" | nix derivation add)