Performs Code Structure/Naming changes for Orchestration Refactor

[kenxu95]

Describe your changes and why you are making these changes

From step 1 of https://www.notion.so/aqueducthq/Building-an-Abstraction-Layer-over-Operators-and-Artifacts-in-our-Go-Code-097d67c45e2f4ffe90185305561d178a, this does this following:

• Rename the database objects in golang/lib/collections
  • WorkflowDAG → DBWorkflowDAG
  • Operator → DBOperator
  • Artifact → DBArtifact
• Fix the layering within our /lib library between lib/collections and lib/workflow. The former should house database-specific struct definitions, and the latter should contain the higher level logic. That is to say lib/workflow should use lib/collections but not the other way around.

Related issue number (if any)

Checklist before requesting a review

• [x ] I have created a descriptive PR title. The PR title should complete the sentence "This PR...".
• [x ] I have performed a self-review of my code.
• [ N/A] I have included a small demo of the changes. For the UI, this would be a screenshot or a Loom video.
• [N/A ] If this is a new feature, I have added unit tests and integration tests.
• [ x] I have run the integration tests locally and they are passing.
• I have run the linter script locally (See python3 scripts/run_linters.py -h for usage).
• All features on the UI continue to work correctly.
• Added one of the following CI labels:
  • run_integration_test: Runs integration tests
  • skip_integration_test: Skips integration tests (Should be used when changes are ONLY documentation/UI)

[cw75]

LGTM