Rewrite Project, remove SourceProject (#274)

* Rewrite project file to have relative paths and drop SourceProject

* Redo project error types

* Tidy up and document Project type

* Strip out init command

src/snapshot_middleware/snapshots/test__project_with_children.snap

@@ -19,12 +19,9 @@ children:
       ignore_unknown_instances: true
       instigating_source:
         ProjectNode:
+          - /foo
           - Child
-          - class_name: Model
-            children: {}
-            properties: {}
-            ignore_unknown_instances: ~
-            path: ~
+          - $className: Model
       relevant_paths: []
       context: {}
     name: Child

src/snapshot_middleware/snapshots/test__project_with_path_to_project_with_children.snap

@@ -20,12 +20,9 @@ children:
       ignore_unknown_instances: true
       instigating_source:
         ProjectNode:
+          - /foo
           - SomeChild
-          - class_name: Model
-            children: {}
-            properties: {}
-            ignore_unknown_instances: ~
-            path: ~
+          - $className: Model
       relevant_paths: []
       context: {}
     name: SomeChild