Otter Generate should deduplicate user-specified requirements with otter-provided ones

[chrispyles]

Currently, otter generate just appends the user-specified requirements to its own list: https://github.com/ucbds-infra/otter-grader/blob/dc50e33aa566202d5a7d92e9f05fecc46e2a4792/otter/generate/init.py#L96

Instead, it should dedupe these requirements, overriding otter's default with the user's (to keep user-specified version info).

Example: User's specified requirements

pandas==2.0.0
otter-grader

This will cause an image build failure currently.