Fix SQL schema imports

[stefannica]

Describe changes

I don't know exactly how, but I ran into a dependency error while running a custom-made script that used the zenml SQL store interface directly:

InvalidRequestError: When initializing mapper mapped class ArtifactVersionSchema->artifact_version, expression 'ArtifactVisualizationSchema' failed to locate a name ('ArtifactVisualizationSchema'). If this is a 
class name, consider adding this relationship() to the <class 'zenml.zen_stores.schemas.artifact_schemas.ArtifactVersionSchema'> class after both dependent classes have been defined.

This was apparently fixed by adding the ArtifactVisualizationSchema import to the schemas root module and removing all direct imports from the root module inside the schema sub-modules themselves to avoid circular imports.

Pre-requisites

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Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Other (add details above)

[fa9r]

It's best practice anyways to specify full import paths, so LGTM and thanks for fixing!