ADAP-1135: Migrate dbt-adapters into a package sub-directory

[mikealfare]

Problem

Now that all of the first party adapters are in the monorepo, we should update dbt-adapters and dbt-tests-adapter to use the same package subdirectory convention as the other packages in this repo.

Solution

• move dbt/ into dbt-adapters/src/
• move tests/ into dbt-adapters/
• move .changes/ into dbt-adapters/
• move root level files into dbt-adapters/ (e.g. README.md, pyproject.toml, e.g.)
• leave copies of README.md and CONTRIBUTING.md in the root, to be updated during the docs PR
• update workflows to remove the package directory workflow
• update all references to dbt-adapters and dbt-tests-adapter to point to the new directories
• update dagger script for dbt-spark to copy dbt-adapters and dbt-tests-adapter files over to the image as well since they get installed locally

CI checks run against this branch:

• Unit tests on dbt-adapters: https://github.com/dbt-labs/dbt-adapters/actions/runs/12995141252
• Unit tests on dbt-snowflake (ref local dbt-adapters): https://github.com/dbt-labs/dbt-adapters/actions/runs/12995153939
• Verify build on dbt-adapters: https://github.com/dbt-labs/dbt-adapters/actions/runs/12995197430

Failed tests:

dbt-spark unit tests and integration tests are failing because we cannot use relative pathing the way that dagger is currently configured in the repo. That will be left as a separate PR. Once this PR is merged, dbt-spark tests will be able to find dbt-adapters in the correct place.