Add src directory structure for dbt-adapters

[mikealfare]

As part of moving to a monorepo, we have decided to adopt the src namespace approach for storing functional code. Additionally, we will move dbt-adapters to be a package subdirectory, just like any other package in this repo. This will allow us to have a place for shared assets, resources, docs, etc. that are not specific to any particular package.

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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