Fix: ensure root package is treated top-level in dbt

[georgesittas]

The added test demonstrates what this PR aims to support. Basically:

• The root project specifies a macros directory in dbt_project.yml that lives within dbt_packages/
• Another package in that directory references said macro without qualifying it with its package name

The above works in dbt, but SQLMesh didn't support it until now because an assumption was made that each package could only see the contents defined within it, not outside of it. So, the package in the 2nd bullet above would fail to see the aforementioned macro and fail at render time, since the symbol wasn't present in jinja_env.globals.