Add src directory structure for dbt-tests-adapter

[mikealfare]

As part of moving to a monorepo, we have decided to adopt the src namespace approach for storing functional code.

While updating pyproject.toml with these changes, I noticed that we use __about__.py to store the package version. However, other packages use __version__.py, and we are locked into this because this version is read by other packages. We should update dbt-tests-adapter to use __version__.py to standardize.

[github-actions]

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[mikealfare]

This is the only file with actual changes; the rest are moves. These changes reflect those file movements.