chore: make csi and controller commands into sub-modules

[pboyd]

Description

Make the controller and csi packages into submodules so we can remove their dependencies from the root module.

This reverses the changes from #601 (for issue #308). The problem back then was that when a dependency of model-registry was updated there would also have to be a corresponding upgrade in csi. But that probably could have been solved with a replace directive in go.mod (see here, it imported model-registry at a specific version without a replace). And secondly, it only needs the generated openapi client which has been it's own package for a while now (without any dependencies), so csi and controller won't actually be importing model-registry itself.

How Has This Been Tested?

Built the images locally and ran tests.

Merge criteria:

• All the commits have been signed-off (To pass the DCO check)

• The commits have meaningful messages
• Automated tests are provided as part of the PR for major new functionalities; testing instructions have been added in the PR body (for PRs involving changes that are not immediately obvious).
• The developer has manually tested the changes and verified that the changes work.
• Code changes follow the kubeflow contribution guidelines.

[google-oss-prow]

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[pboyd]

/cc @Al-Pragliola