Move the entirety of dbt-tests-adapter into the dbt-tests-adapter directory

[mikealfare]

Problem

hatch makes a lot of assumptions about where files are relative to each other. In particular, it prefers one package per repo.

Solution

Instead of nesting the namespace packages together, move the dbt-tests-adapter package into its own directory, isolated from everything else.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development, and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX

[github-actions]

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