Enable errors for Private and Protected nodes

[noel]

Is your feature request related to a problem? Please describe. Right now, upstream producer private and protected nodes are not included in the consumer project. When a user tries to access one of these nodes, they are given an error that the node does not exist. It would be a better UX to show an error like

Parsing Error
  Node model.great_bay.test_us_population attempted to reference node model.balboa.us_population, which is not allowed because the referenced node is private to the marketing group.

[nicholasyager]

Thanks for creating this issue, @noel! I've done some initial digging and here's what I've found so far:

• dbt-loom currently (0.4.1) purposefully only injects models with public access into a project. This is mostly arbitrary and can be easily updated.
• If we remove this protection and then ref a private model from an upstream project, dbt-core will blindly accept the ref and no compilation error will be raised. Interestingly, it looks like dbt core falls back on standard package restrict-access logic for checking if a model ought to be accessible by a consuming project.
• There is a check for dependencies as part of the restrict-access logic, but I've not yet found how dependencies are loaded. My current theory is that dbt Cloud uses undocumented aspects of the dbt runner python API to inject dependency information.

I believe that there may be some ways to work around this through some clever python patching -- either by wrapping the access code with some custom methods to verify dbt-loom access, or by patching how dependencies are loaded so we can inject dependencies in, too. In any case, the investigation continues.