[flytekit] Test pytest with --import-mode=importlib

[wild-endeavor]

Describe the bug
See the slack thread. Running pytest with --import-mode=importlib causes tests to fail in certain cases. We should find out why, and fix.

To Reproduce
See slack thread for error. Basically the detection of nested tasks is incorrect when this flag is set.

[eapolinario]

Relevant pytest issue: pytest-dev/pytest#7245

I could reproduce the issue in pytest==6.2.5, but not in 6.3.0 which contains pytest-dev/pytest#7870.

There's talk of making importlib the default import mode (as implied by the pytest docs), but it will probably be a while until that happens. In the meantime, it's safe to close this issue.