[CT-3468] allow creating unit testing fixtures in non-root projects

[graciegoheen]

Is this your first time submitting a feature request?

• I have read the expectations for open source contributors
• I have searched the existing issues, and I could not find an existing issue for this feature
• I am requesting a straightforward extension of existing dbt functionality, rather than a Big Idea better suited to a discussion

Describe the feature

Right now we assume that all file fixtures for unit tests are in the current root project. It's possible that people might want to provide reusable fixtures with their packages.

Describe alternatives you've considered

Not allow folks to define fixtures in non-root projects

Who will this benefit?

No response

Are you interested in contributing this feature?

No response

Anything else?

No response

[gshank]

One of the things that we would have to decide how to handle, is how to specify an external package fixture. "otherpackage.fixture_name"?