Mark project file as instigating source for top-level instances

src/snapshot_middleware/snapshots/test__project_with_unresolved_properties.snap

@@ -5,6 +5,8 @@ expression: instance_snapshot
 snapshot_id: ~
 metadata:
   ignore_unknown_instances: true
+  instigating_source:
+    Path: /foo/default.project.json
   relevant_paths:
     - /foo/default.project.json
 name: unresolved-properties
@@ -13,4 +15,4 @@ properties:
   Value:
     Type: String
     Value: Hi!
-children: []
+children: []