Change JSON model naming behavior.

- JSON model names now come from the file name
- The 'Name' field is now optional for the top-level instance
- Snapshot tests run way faster by executing Rojo directly instead of Cargo

rojo-test/build-tests/json_model_legacy_name/default.project.json

@@ -0,0 +1,6 @@
+{
+  "name": "json_model_legacy_name",
+  "tree": {
+    "$path": "folder"
+  }
+}