Fix: Include root package in search candidates when resolving dbt macros

[vchan]

dbt package macros can be overridden in the root package. This PR includes the root package when searching packages during macro dispatch resolution.

[georgesittas]

So, to try and summarize: when there's a macro namespace, we now always check the root package first for any possible overrides. And this is bound to change when we take dispatch configs into account.