directory CONTAINED file relationship not working

[pcoccoli]

Describe the bug
FIND directory CONTAINED x (when x is a file variable) returns nothing but FIND directory LINKED x for the same x does return.

Details of the bug
The problem stems from relation to STIX property resolution when the STIX property is a reference list. In this case, STIX defines a directory:contains_refs property. For reference properties that are NOT lists (i.e. end in _ref and point to a single object rather than a list of objects), Kestrel will first check if the property exists in the current session. For ref lists, this isn't done.

To Reproduce
Unit test to show the problem:

def test_find_dir_contained_file(set_empty_kestrel_config, proc_bundle_file):
    with Session() as s:
        stmt = f"""
                files = GET file
                        FROM file://{proc_bundle_file}
                        WHERE name = 'svchost.exe'
                dirs = FIND directory CONTAINED files
                """
        s.execute(stmt)
        files = s.get_variable("files")
        print(json.dumps(files, indent=4))
        dirs = s.get_variable("dirs")
        print(json.dumps(dirs, indent=4))
        assert len(dirs) == 1

This test will fail if the "contains_refs" property doesn't appear in the STIX bundle (it usually does not). However, it should pass since file:parent_directory_ref is there.