Migrate to dbt-common + dbt-adapters for v1.8+

[jtcohen6]

Discussion:

• Adapter/Core Decoupling Migration Guide dbt-labs/dbt-adapters#87

tl;dr: Starting with dbt-core v1.8, adapters should no longer import dbt-core directly, but instead import a set of common interfaces defined in dbt-common + dbt-adapters.

I wanted to give this one a try :)

It's mostly a matter of switching import statements. I found two utility classes — SourceConfig and TargetConfig — which depend on type definitions (RuntimeConfigObject, SourceDefinition) that are not included in our common/adapter interface. I'll open a draft PR so we can take a look together & discuss.