Port the adapter macro to python

[beckjake]

resolves part of #2302

Description

This ports adapter_macro to python, which involved some cleanup of the node selection logic.

I split the macros and manifest+query header contexts out from the configured context as that file was getting too busy.

I made no change to actual behavior, so the real test here is that all the existing integration tests pass.

I added a get_adapter_type_names to the adapter factory to handle an inheritance-style lookup, but that looked like it broke things. I rolled that back but would like to keep the adapter factory changes as I think they're probably for the best.

In the future, we should consider changing it so redshift automatically tries the postgres version of a macro when there's no redshift version before falling back to the default, but let's keep the breaking changes to a minimum for now.

A future PR will implement the dispatch name and additional arguments.

Checklist

• I have signed the CLA
• 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
• I have updated the CHANGELOG.md and added information about my change to the "dbt next" section.

[jtcohen6]

Looks good! I ran the dbt-utils integration tests with this branch, and all went according to plan. My only comment is around the error message returned when adapter_macro fails to find its match.