Expand search paths for schema.yml files

[beckjake]

Describe the feature

Currently, dbt searches for schema.yml files in source-paths, data-paths, and snapshot-paths. We should also search in macro-paths, and analysis-paths since macros/analyses can be documented now.

Describe alternatives you've considered

We can leave it like it is and tell new users over and over again that they need to put them in one of the three special directories. I think that's pretty unintuitive.

Additional context

I'd really really like to get this in for 0.16.0. It's a super easy fix (Add stuff to project._all_source_paths, change 2 calls) and it just seems silly to not do this.

Who will this benefit?

• New dbt users
• People who respond to support requests in slack