Fragment: project() on already projected fragment yields incorrect result

[sighingnow]

Describe your problem

PR #239 (and alibaba/GraphScope#211) introduced the "general project" feature to property fragment, but it yields incorrect result when when do project() on an already projected fragment. E.g.

def test_projected_on_project():
    graph = load_ldbc()
    pg1 = graph.project(vertices={"post": [], "tag":[], "tagclass": []}, edges={"hasTag": [], "isSubclassOf": []})
    logger.info("pg1.schema = %s, %d", pg1.schema, pg1.vineyard_id)
    pg2 = pg1.project(vertices={"tagclass": []}, edges={"isSubclassOf": []})
    logger.info("pg2.schema = %s, %d", pg2.schema, pg2.vineyard_id)

We currently get:

2023-03-01 18:03:19,266 [INFO][test_flatten:16]: pg1.schema = oid_type: LONG
vid_type: ULONG
type: VERTEX
Label: post
Properties:

type: VERTEX
Label: tag
Properties:

type: VERTEX
Label: tagclass
Properties:

type: EDGE
Label: isSubclassOf
Properties:
Relations: [Relation(source='tagclass', destination='tagclass')]
type: EDGE
Label: hasTag
Properties:
Relations: [Relation(source='post', destination='tag')]

... 

2023-03-01 17:33:27,648 [INFO][test_flatten:18]: pg2.schema = oid_type: LONG
vid_type: ULONG
type: VERTEX
Label: post
Properties:

type: VERTEX
Label: tag
Properties:

type: VERTEX
Label: tagclass
Properties:

type: EDGE
Label: isSubclassOf
Properties:
Relations: [Relation(source='tagclass', destination='tagclass')]
type: EDGE
Label: hasTag
Properties:
Relations: [Relation(source='post', destination='tag')]

Which is wrong.