[modin] Append to path to avoid namespace collision on development branches

[devin-petersohn]

What do these changes do?

Allows development branches to run Modin.

Related issue number

Resolves #3611

[robertnishihara]

Hm, this is the behavior we want if Modin is imported first, but not the behavior we want if Ray is imported first.

I guess the problem comes from the fact that on the workers, Ray is always imported first.

[richardliaw]

Would a soft import help here too? ie, try import modin and if not found, then append to path?

[robertnishihara]

Ideally we'd have two different behaviors.

1. If ray is imported first, then import modin will only find the copy of Modin that is shipped with Ray (@richardliaw I think your proposal violates this).
2. If modin is imported first, then we only find the copy of modin that is first on the PYTHONPATH.

Unfortunately I don't see a good way to do these both simultaneously.

[richardliaw]

Oh; sorry I guess I'm missing context. When do you want the first behavior? Why not have ray.modin separate from modin (which holds its own in site-packages)?

[AmplabJenkins]

Test FAILed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/Ray-PRB/10334/
Test FAILed.

[devin-petersohn]

@richardliaw - The goal was to make this similar to how pyarrow is handled in Ray. The challenge is that Modin depends on Ray. It can cause the version of Modin being imported on the workers to mismatch the version on the driver, which is especially problematic for releases where we have made significant changes to the code that runs in Ray.