Test collector bug when --rootdir is not real path #9008

(.venv) $ pip list
Package    Version
---------- -------
attrs      21.2.0
iniconfig  1.1.1
packaging  21.0
pip        21.2.4
pluggy     0.13.1
py         1.10.0
pyparsing  2.4.7
pytest     6.2.4
setuptools 41.6.0
toml       0.10.2

(.venv) $ tree
.
├── mypytest
└── tests
    ├── __init__.py
    └── test_dummy.py

1 directory, 3 files

(.venv) $ cat mypytest
#!/usr/bin/env python3

import sys
import pytest

class MyCollector:
    def __init__(self, logfile):
        self.logfile = logfile

    def pytest_collection_finish(self, session):
        with open(self.logfile, "w") as f:
            for item in session.items:
                f.write("%s\n" % dict(nodeid=item.nodeid, fspath=item.fspath))

if __name__ == "__main__":
    plugin = MyCollector("./collector.log")
    pytest.main(sys.argv[1:], [plugin])

# chdir to "tests"
(.venv) $ cd tests
(.venv) $ pwd
/var/tmp/pytest-issue/tests

# this is good
(.venv) $ ../mypytest --collect-only --rootdir /var/tmp/pytest-issue > /dev/null && cat collector.log
{'nodeid': 'tests/test_dummy.py::test_plus', 'fspath': local('/var/tmp/pytest-issue/tests/test_dummy.py')}

# bad if rootdir is not realpath: let's run from /var/tmp/foo which is not realpath.
# fspath is good but nodeid has not the "tests" subdir.
(.venv) $ ln -s /var/tmp/pytest-issue /var/tmp/foo
(.venv) $ cd /var/tmp/foo/tests
(.venv) $ ../mypytest --collect-only --rootdir /var/tmp/foo > /dev/null && cat collector.log
{'nodeid': 'test_dummy.py::test_plus', 'fspath': local('/var/tmp/pytest-issue/tests/test_dummy.py')}